Welcome to the project :)
Most important is the number and clock speed of the shaders. The other parameters like vram and core speed have only little impact on the calculation time.
Please check the following thread in the FAQ section:
http://www.ps3grid.net/forum_thread.php?id=316
You can buy all the cards marked as green, according to the amount of money that you want to spend.
A good start would be a 8800/9800GT (its the same card basically, the later one is just relabeled), it should still give the most credits per R$, € or $ :D
regards,
koschi |