If your primary concern is being able to maximize upgrading potential, your only real option is to build it yourself.
HP/Compaq systems will have as many integrated components as possible, and HP/Compaq has always been along the bottom of the barrel quality wise.
Dell has been in a state of perpetual decline for several years now. Whole batches of particular lines can have massive problems. They're still better than almost everyone else on both quality and upgrade ability, but you're still going to severely limit your upgrade options if you go with Dell.
You could try plunking down the extra cash for one of Dell's Alienware systems. Back before Dell bought Alienware, they were probably one of the best pre-made systems you could get for upgrade potential. All because they were focused on high end gaming, and that didn't allow for using cheap low-end parts like everyone else will do any chance they get. I don't know how much damage Dell has done to the Alienware brand since taking over the company however. They might have left it completely in tact, they might have completely dismantled it, and tried to commoditize things.
One thing I can whole heartedly recommend from Dell, are their LCD monitors. IMO, they cannot be beaten at the 20" and above level. And while their contrast ratios might seem lower than competitors, Dell's ratios are probably the most accurate of anyone's. If they say a display has an 800:1 contrast ratio, you're probably going to get something very close to an 800:1 ratio. They don't claim a 1000:1 ratio, and only deliver like a 750:1 ratio. Outside of their LCD monitors, I'm not a big Dell fan after spending about a year and a half supporting almost nothing but Dell Optiplex systems.
With any brand name system, you'll have very limited CPU upgrade options, RAM capacity will be limited, FireWire is probably out of the question unless you waste a PCI slot to add one in, and you tend to either get integrated video (which siphons off your system RAM) or have to pay like a 25% premium over retail for some very high end video card.
In the end, nothing will ever beat doing it yourself. Spending time researching various parts to go into the system, and watching for sales on any of the parts you want to include, so you can save money without sacrificing quality. If you take your time, you should be able to upgrade your system in perpetuity. Aside from the floppy drive, I don't think there's a single original part left in this system I built about 2 years ago. I've replaced the motherboard twice, CPU twice, RAM, video card, case, power supply, HDDs, one of the DVD burners might be "original" come to think of it... This is all way more than you could ever expect out of a brand name system.
Moral of the story is, you're going to have to do it yourself. You've got a few months before you wanted to purchase anyway. Time to hit the hardware review sites and start reading. Start with the most recent motherboard review guides, and work your way out from there. Then watch deal sites and store ads for good deals on what you want.
My wife and I would like to get a new computer with Vista installed. We are looking to purchase in a couple of months. We specifically want a computer than can be upraded in the future. Here are the specs:
Intel Duo-Core 1.86 or 2.4
2 or more GB of RAM that can be upgraded
250G or more hard drive
Multiple USB ports, including FireWire
A good video card that does not use memory(non-Turbo Cache)
17" or larger monitor
MS Office suite: SB or higher
This a stripped down list. We are not gamers and need a computer for our digital pictures, iPod files, Office applications, and general surfing.
HP seems the most complete, however, I am not keen on their keyboard(seems chintzy). Compaq is barebones, although made by HP. Dell seems a good system with a nice keyboard but am concerned(along with Gateway) about service issues.
The operative word here is upgradability. We want a computer that can keep up with future trends and can have parts added/swapped as needed.