Here's a handy dandy FAQ from the GPUGrid BOINC project.
http://www.gpugrid.net/forum_thread.php?id=316
I have a 9800 GT, but I use it for the Folding@Home GPU client. In general, more memory is good (most OS's are starting to offload the display to video memory), and more pixel shaders is good. They way these cards work is actually like the Cell processor. You have a central control processor, then lots and lots of pixel shader units. Each shader unit is a tiny, highly specialized processor. Between the different models, instead of making major improvements between the primary processors, they just add more and more pixel shaders (and often more memory).
I do recommend... if you run this, make sure the computer has good circulation, and make sure the video card has free space around it for air to circulate. Also, every month or two, power everything down and blow the dust out with compressed air. The gamers at work tell me that every couple months, they can see the difference when they clean the card's heat sync and fan.
Another thing, and this is important, check the power supply's wattage against the max draw of the video card. Newer video cards have some crazy power requirements, and most dual card systems have power supplies ranging from 1 kilowatt, to 1.25kW or 1.5kW. Read the issues with older Folding@Home clients. Basically, they had no internal throttling, ran the card at 100%, and for some people this was overdrawing against the power supply and shutting down the machine (non-gracefully, obviously). You will notice on most video card packaging now, on the system requirements, the power supply wattage is in the large print section, along with processor speed, and no longer in the fine print.
Hope that information was somewhat useful.