[Fyron]

1) According to the Ars system guide, the 8800GT 256MB seems to be the closest Nvidia cost/performance competitor in that price range. The 8800GT 512MB would possibly be a better long-term investment, for about $80-100 more. The Geforce 9 series cards are coming out soon, so it might be best to hold off on a video card purchase until they are out. The 8 series cards should see price drops at that time. The integrated video on the motherboard you linked won't be ideal for playing newer games, but it will be fine for older games and SE.

2) Yes.

4) 500 watts is overkill for pretty much any PC that doesn't have a SLI video card config, so you will be fine.

As for RAM's type, one way to tell is to look up the spec sheet of your current motherboard (any given motherboard can generally only use one type of RAM, with occasional boards sporting some legacy slots of the previous generation). If you run the "msinfo32" application, you can see what your motherboard is under the first page (system summary). It will be called something like "System Model."