Building a new computer

I've recently fallen into the sticky situation of having to build a new system from the ground up, and fast.  I'm hardly an expert at working with hardware, but have worked with my old system a bit and am pretty familiar with what to do.  Anyway.  My budget is going to be around $500, unfortunately, so I'd like to know what's the best system I can put together for that much.

I already have:
Monitor
Keyboard
Mouse
Case
Asus P4S800 Mobo

....aaaand that's it. :(  I know this is going to be highly challenging or perhaps even impossible at this price range, but I'd ideally like to end up with a system that can at least play WoW decently and NOT have it look like COMPLETE shit.  Lots of RAM is also essential as I am a video editor.  Any and all suggestions are welcome, or even partial solutions if it's not possible to afford all parts for $500.

The ram and that card is why everything ran as shit.
Need at least 2x512 DDR sticks with half a decent processor 2ghz minimum with a good graphics card like 6600GT PCI-E/AGP to play recent games without sacrificing too much quality.

I played FEAR with high graphics on a Pentium 4 1,5ghz smoothly using a 6600GT.. Oddly I got VERY high frame rate too compared to most other games, but there were jerks in the game now and then because my system had slow SDRAM instead of DDR and not really a very good motherboard.

As for HDD, Maxtor drives are rather fast but noisy and also hit and miss on the QC. If you get a good one it will work. Western Digital are worse. Seagate seems to be better.