If you are getting a Dell XPS
Why not the XPS Gen 5 (link floating around somewhere here, you could always google it too) it's in the neighborhood of $1700-$2000, but features the Dual Cored Pentium Extreme Edition 840 (if you REALLY wanted it, as Alienware charges nearly $3000 for that CPU (with a decent level of other hardware though (i think you get a 6800GT in that system, but not entirely sure)
But i'd like to suggest avoiding all three of those systems, and just building it yourself. You are on a tight budget, want performance and should realize that building it yourself will save you between $100 and $600 depending on what you build vs. what you could buy.
You can build a dual 6800GT system for roughly $1600, if you really feel that you would use all that power.
The comment I have is that SLI is not worth the investment, I purchased a GeForce 6800GT after being sick and tired of various games either:
A) not running in full DirectX 9.0
or
B) running extremely slow
on my GeForce FX, so I purchased a 6800GT...
At first I didn't think it'd be so amazing in terms of power, I mean I knew the cards specs inside and out, and I know the human eye captures at between 29 and 32 FPS, so if the game is running at 40 FPS your perfectly fine in terms of performance, and the 6800GT did most games at 40-140 FPS in reviews i'd read, but I was thinking "meh, my GeForce FX does most games at 40-70 FPS"
well, what I forgot to factor in, the 6800GT is doing 40-140 FPS WITH Anti-Aliasing and Anisotropic Filtering, and VERY HIGH in game quality settings, plus a fairly good resolution
well was I in for a treat, the thing just tears games limb from limb
I would suggest DIY building your own machine, wait about a month and a half, as AMD releases Athlon64x2 on the 31st of May...
Since Athlon64x2 chips are Socket 939, they will run in any existing Socket 939 board (the board has to be Athlon64 FX capable, most are, even the middle range/semi-low range boards (i'd say spending between $70 and $140 on the board would be good (if your getting SLI between $170 and $200 is fair))
I would then get an Athlon64x2
just pick a model which fits your budget, as all of them are rather fast
along with an nForce4 equipped motherboard sporting PCIE, then i'd grab a PCIE 6800GT, a decent sized hard drive, 1GB of DDR and any other features you want
for power i'd rely on an Antec NeoPower 480W
as it provides stable rails, it's modular (you only use the connecting cables you need, so you have better cable management) it features Active PFC (helps keep the power more stable) and it can provide enough juice for all your hardware needs
dual 6600GT's will not (in real world) outperform 6800GT's and 6800U's
in benchmarks they will usually win, but in a game like Half-Life 2, the 6800U or 6800GT will easily win
Another option for video cards is the ATi Radeon X800XL, which is slightly faster than the 6800GT (it's between the 6800GT and the 6800 Ultra) and it doesn't cost much more
but it's purely your choice (it's basically an nVidia vs ATi battle, both cards are blisteringly fast, so either would be an excellent choice)
I would also suggest a good CRT (a nice 19" Flat one)
possibly an LCD if space is an issue, but LCD's of the large variety are still expensive
but building your own would probably be best
and having a dual core AMD with a 6800GT or X800XL would just scream in games...
the one thing i'd suggest avoiding is buying a system with nothing but the intention of gaming
couple the system's purpose with something more real-world, or else you'll end up spending $6000 over a few years in upgrades/new systems just to keep up with the times...
for example, I do game on my PC
but I also use it for office work, music editing, music listening/storage, video watching, web browsing, IM, basically it does everything
I have two monitors to optomize productivity in office work, and so that I can have my music on the 2nd screen (yes, currently (as i post this) i'm using a 17" CRT running 1152x864 just to monitor play on a CD (it normally does more, lol))
i'm just trying to explain that going for top performance is like chasing a ghost, you'll never actually catch it, because someone is always going to have something bigger and better, and it'll end up costing you a fortune trying to pass them
and i'll leave you with the following thought
buying a PC entirely for gaming is a waste in my opinion, it's a waste because your spending thousands of dollars to play software costing $40-$50
consider Xbox or Playstation 2 (not to buy, but just think about them for this example) which only cost around $149, or the Gamecube which is just under $100
they cost between $100 and $150
and all they do is play games
say you buy an Xbox, you purchase 4-5 games and you've already done the system's value in gaming, and if they are decent games (let's just use the examples of Halo, Halo 2, Doom III for Xbox, Grand Theft Auto Vice City and Tom Clancy's Splinter Cell, all of which are decent games, all of which (except Halo 2) are avliable for PC)
but let's consider that
buy those games, and the Xbox
it's around $300
and the gaming experience is worth that, your paying around $45 a game (i'd say average (i'm talking Chaos Theory for Splinter Cell btw) and you are getting a decent gaming experience
but consider that for a PC
say you buy a $1500 Alienware
and those 5 games
which total around $100
your going to pay $1500 to play $100-$150 in software with no other main goal for this $1500 device to achive?
you could buy Xbox, do the same thing, and save yourself around $1300
i'm just saying think about the moitivation to buy the computer
if your just going for gaming, a console is what you really want, because that is all a console does
yes they will have slightly lesser graphics, but graphics don't make a game
and most games on Playstation 2 and Xbox look pretty good, I cant comment on Gamecube (i've only seen maybe 5 minutes of game play on the GameCube, and it was just an adapted N64 game, which looks the same on N64, so it wouldn't be fair for GameCube to have it's graphics compared to N64 (because I know it's faster than that))
well, good luck shopping
and if you would like help finding parts i'm usually avliable on cnet (or e-mail)
and i can provide a suggested parts list
also, i'd suggest looking at www.newegg.com
for all your parts
they are an excellent retailer with good prices (www.zipzoomfly.com is another good option)