I haven't bought a game in the last 12 months due to a bad mobo choice last build. I'm planning on turning my current multiple-use desktop into a server for the home network, and replacing it with a brand-new gaming machine. I've got a single, mid-range Nvidia card that I hope to use in some kind of SLI config, but like it says, multi-GPU is not required. I've already got the drives and Vista 64 license, and want to just rebuild my current machine better, faster, stronger (and maybe smaller).
My current machine has a crappy-ass M3N78-Pro mobo that only supports one single channel of memory, only at 800mhz, and only up to 4GB. It can run Mass Effect 1 at medium settings, L4D 1 and 2 at high settings, and Sins, Civ4, and SimCity4 at low, on account of the crappy memory. It'll handle modded Oblivion at 'Very Low.' Basically, I just want a gaming rig that can keep games running for the next few years, with room to upgrade the graphics/processor 2-3 years down the line.
I've made 5 builds so far, mostly budget workstations for friends and family, and no really high-end stuff. Multi-GPU virgin, but curious. Have had bad experiences with ASUS mobos, and have gotten awful bang-per-buck with Intel and ATI (which sucks, considering license restrictions). Looking at a few MSI and Gigabyte boards with x16/x16/x8 at the very best, x8/x8 at the most likely, and x16 mATX if I can find a video card that's cheap and worthwhile. As for chipsets, I'm looking almost entirely at the dual- and quad-core Phenoms (If I had the money, I'd maybe spring for one of those fancy X6's, but I don't). Memory is 8GB minimum of DDR3, DDR2 if I absolutely have to keep it in-budget.
I'm not ready to jump onto Windoze 7 yet, and am fine with only Dx10.
$500-ish will get you an AMD system with a better video card than your 9800GT.
Athlon II X3 450: $80
AS Rock 880GM: $60
HIS HD5770: $130 (w/ $20 MIR)
Mushkin 4 GB DDR3: $40
WD 500 GB HD: $40
Antec 400W Neo Eco: $45
Rosewill R101: $30
cheap DVD burner: $17
Extra fans: $8
$450 total before rebate. a GTX 460 768MB model would be $30 more