My suggestion for building is to wait for the post-Christmas and New Year's sales. I built a low-to-midrange gaming PC right after Christmas 2010 and got what was $800 of components at the time for around $560. The machine is still holding up solidly today for what I want to play.
I'd suggest a.) putting together an approximate wish list, then b.) waiting for sales, and c.) if one of your preferred components isn't on sale replacing it with something from a sale list. (Thus, approximate wish-list). Minor differences in RAM or video card manufacturer won't matter as long as you're smart.