Whatever you do, try to get a card that will take the 7620 GPU units. They're VERY demanding and will push your cooling to the limit, but give me around a 3-5k boost using GPU3 (Fermi).
What's the minimum card that will take those? I'm not much of a gamer, so I'd love NOT to drop 3 bills on a GPU. I was thinking GTX 550 Ti...
GPU folding is all about the "Processing Cores" (programmable shaders). The more you have the faster you'll go.
The GTX 550 Ti has 192 cores for ~$130 (Newegg)
The GTX 560 has 336 cores for ~$190 (Newegg)
The GTX 560 Ti has 384 cores for ~$220 (Newegg)
Here's a recent-ish comparison of folding cards: http://www.bit-tech.net/hardware/graphi ... -folding/4
Please note the 560 (non-Ti) was not released at the time of writing, and nVidia's non-Ti/GTX/FTW/BBQ editions of their mid-range gamer cards are typically the PPD/watt champions. To put GTX 560 folding in perspective, the average reported PPD is generally in the 11k-13k region stock. My stock GTX 570 (a $300 card, 480 cores) ran ~14-15k ppd.
My comment regarding 7621 units has more to do with cooling than speed. These units take my watercooled GTX 570 to 66c with my fans on high. There is no other unit that exceeds 54c with my fans turning ~200 RPM slower all around. They're INTENSE. So, when you're considering a card for these units, pay special attention to both the card's cooler (you want a decent one with good acoustics) and your case ventilation.
One other thing: these second-gen Fermi cards are excellent overclockers with a little bit of voltage (clock speed + more cores = more PPD). Voltage = heat, so if you plan on OCing your card for a PPD bump, get a card that sports a monster cooler. Of course you can always go aftermarket...
One more note on OCing your card: My PPD jumped ~3k (from ~14-15k PPD to ~17-18k PPD) when taking my card from 797 MHz to 920 (1594 to 1840 MHz on the processing cores), so PPD gains can be dramatic for not a lot of effort.
All things considered, I'd strongly consider this card for the factory OC and cooler ($10 for 90MHz and a nice quiet heatpipe cooler seems like a good deal): http://www.newegg.com/Product/Product.a ... 6814125390
Also, keep in mind you'll want to run some sort of load balancer for max PPD. I'm using process lasso to limit the GPU folding to two of my CPU cores so it doesn't leech too many CPU PPDs, but there will be a loss.