about a decade of experience in high performance personal computers, overclocking, etc
I can say build it your self, you'll get a faster, nicer running box than buying from Dell or HP (and from about a decade of experience in IT, I can say that I will happily NEVER own an HP branded computer, I've no issues with (most of) their printers, scanners etc).
The reason people buy Dell is twofold:
A) theres the lack of knowledge/information/self-confidence/whatever else to build a system themselves
B) elitism, its the same reason your rich boss buys a big Mercedes for his birthday, just to show off that he can throw down the money and drive off the lot in $100,000+ of motor vehicle, basically Dell XPS, or Alienware (especially the ALX systems), anything from VoodooPC, etc, they exist to cater to the so called "gamer market", which is basically marketing amazingly over-priced hardware to people willing to pay anything to feel like they've gotten the Ferrari of computers...this quite simply isn't the case
Yes, you can spend easily in excess of $10,000 on a Dell XPS, and easily in excess of $20,000 at some of the more unique boutique shops (VoodooPC especially, although HP may have changed that), most of that usually going into cosmetics (like color-matched peripherals, etc).
Look at computer price as a measure of years out of the system, capped at about $2500 equating to 2.5 years
If you spend $1000 or so on a new DIY computer, it'll be competent for about a year (before someone starts screaming how their 2-3 year old box still works, what I'm saying is that a $1000 DIY system will run everything on the market at its time of production, all the way up to a year from its time of production, at which point you will likely see a performance defficit (usually in gaming) where an upgrade starts to be attractive, once you clear ~$2500 you're just wasting money on the system, given that no computer will outlast ~3 years and still be in the top tier)
So, I get that you want a top end capable gaming machine, now consider this:
My current gaming system cost less than $1000, I believe the total price is somewhere around $750-$900 (haven't actually priced out components that I recycled from previous builds, however that $750 figure accounts for the new parts, and the $900 should cover everything else had I bought it new).
Its specs are as follows:
DFI LanParty UT RDX200CF-DR
AMD Athlon64 x2 3800+
2x512 G.Skill FX TCCD DDR
2x512 Patriot PC3200 DDR (or 2GB total, for those of you keeping track at home)
PNY Verto GeForce 7900GS 256MB
2x80GB ATA-133 HD's (this is a point of contention, most people want more space, I don't blame them, I just don't use that much hard-drive space so this is more than suitable for my needs)
and some various peripherals like keyboard, mouse, etc
The above runs every game on the market, no questions asked, with more or less any settings I want excluding 1920x1200 and higher resolutions, and HDR + AA at the same time in non-Valve games (not a performance issue, an architecture issue of the NV4x and G7x chipsets, the 7900 is G71 for reference, Radeon X1800/X1900 and later are all capable of HDR + AA, and the 7900 does have enough processing power to handle it, its just an architecture thing, GeForce 8 can also do HDR + AA).
So, I'm hoping my example helps put it into perspective, you dont need to spend Ferrari prices, to get Ferrari performance, because there simply doesn't exist "gaming" hardware, its just the same stuff on the inside.
Honestly, I'd look at www.newegg.com or www.zipzoomfly.com, check out whats available, and look to spend max $1400 (monitor is always extra) on your new system, re-use what you can from previous builds (optical drives for example, 16x DVD-ROM drives will be 16x DVD-ROM drives until the end of time, and why spend ~$20 when you likely have a few laying around).
I'd look at quality internals, and you'll get a quality experience, if you buy junk, it'll give you junk. Look to a ~$200 per part maximum, although you'll generally be spending no more than $150 on any single part (exceptions include the graphics card and the monitor). Go ahead and buy the 8800GTS if you want it, nothing wrong with an indulgence like that, but don't think it requires $4000 of hardware around it, and please dont' be duped into believing that you gotta buy the absolute highest end hardware on the market to get a good experience.
A little secret of these big-box boutiques:
Most of their ~$5000-$8000 PC's cant hold a candle to what you could custom build for ~$2000, thats why they've finally broken down and restorted to factory overclocking, to try and regain some market share, a system you hand-build for $4000 isn't neccisarily faster, its just wasted money to a degree, unless you know what you're doing with that type of hardware (super expensive mainboards are expensive for a reason: BIOS options and overclocking, no other reason you buy a $300 mainboard unless its going in a server).
Thats my 2 cents, hope it helps some.