I have one system with an nVidia GF 8500 GT that I have running Aqua, QMC, SETI and PrimeGrid (usually only on PG for challenges). That card isn't double precision. I'm not sure what they mean about Compute Capability 1.3. This motherboard says it has 2 x PCI-E x 16 and 2 x PCI-E x 1. Does this mean I can add another card and run two? This is an ASUS P5K Premium.
I have another system with an ATI Radeon HD 4650 that runs MilkyWay, Seti, Rosetta and PrimeGrid (also for challenges). That card isn't double precision either.
My old XP system has a GeForce 7600 GS so it won't run any of the GPU stuff.
The second and third systems are AGP. Not much capability to expand there.