As far as actual loading time, it's the processor (and memory, to a lesser extent) that's going to affect those the most.
If you've got someone computer geeky available to you, I'd suggest one of the lower-end quad core processors in a custom system - I understand the Intel Q6600 is quite a bit cheaper now than it used to be, and it's certainly got enough balls to do what you need it to. As far as memory goes, DDR2 is quite sufficient, and 3 GB will top out a 32-bit system.
Run XP on the system, of course - it's got quite a bit less system overhead, and although it's -less- of a worry when you're kicking 3 gigs of RAM, you're typically going to want to squeeze every drop of performance you can out of a gaming rig.
Lastly, get a decent-to-good video card - 512MB of video RAM will do quite enough for most people's uses, even when gaming, and the prices on those are quite reasonable. If you're not doing a custom built system, you'll probably want a video card on the side - they typically come with either lower-end video, or craptastical Intel 965 chipsets, neither of which will be worth a damn on a gaming system.
For the record, I built my own system for just under $900. Add in the $200 worth of parts I "recycled" (from the old system), and take your own opinions:
AMD Phenom 9600 processor, 4GB RAM, 512MB NVidia GeForce 8600GT (I think it's the GT model...), 2 DVD drives, 200GB hard drive, Thermaltake V1 Fan (this thing is GOD - link at the end) , Ultra case, motherboard of some sort (not overclocking it, so it's not a worry)... and I think that's it.
The Thermaltake V1 fan looks like this:
http://www.etmoonshade.net/bigfan.jpg - it stands just a bit shy of 6 inches off the surface of the motherboard, but it keeps the processor niiiiiiice and cool. It -is- too big for a lot of cases, so be careful if you get it - get yourself a full tower sort of case, or a wide mid-tower, or you'll end up not being able to fit it. Any other questions, poke me, prolly on IRC.
