The game is running slowly even on a good system.

Hey, I just bought a video card from SAPPHIRE called ATI Radeon HD 2900 PRO with 1 GB of memory. I'm also planning to buy a new motherboard for it. My question is, how much should a motherboard for this video card cost, and as far as I know, each motherboard supports a minimum amount of video card memory, right? And what frequency should I also buy for the video card? If you don't know, I already know your answer: go and consult with a store, but I really don't feel like going there again.