Well, when I put together my two gaming rigs in 2006-2007, with the original E2140 CPUs, I bought ATI X1950 cards, because I got great deal on them, and at the time, they were half-decent for gaming.
Later on, I got some DFI X48 mobos, and some Q6600 CPUs, and then I bought four HD4850 cards during release week (well two of them anyways, and two more a month later). Primarily, because they were the latest and greatest on the market, and I could get them from BestBuy B&M with a 25% discount. For brand-new products on the market, I thought that was a great deal.
But then I never built the X48/Q6600 rigs for a couple of years, so the HD4850s sat in their boxes for a few years.
I bought a couple of Q9300 CPUs a year or two ago to upgrade my two rigs, and then I picked up a pair of GTX460 1GB OC cards. Primarily, because one of my intended uses was Folding@Home, and gaming was secondary.
Now, I've been using the Q9300 CPU OCed to 3.0Ghz, and GTX460 1GB OCed to 820 or so, for PrimeGrid and other BOINC projects.
Within the last month, I picked up a ASrock 990FX board with three PCI-E x16 physical slots, and a hex core X6 1045T Thuban, which I OCed to 3.51Ghz. Plan on running my GTX460s in SLI, and adding a third GPU for PhysX and CUDA.
Still trying to decide on that third GPU.
In the future, will I pick NV or AMD? Who knows. If more DC projects start to support AMD's GCN architecture, then I would likely pick some of them up. Otherwise, I'll get a CUDA card with the best price/performance.