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.

I had a cheapass Celeron processor in my old system, and everything seemed to run like crap...I had it comboed with 256mb of RAM and a GeForce FX 5200, could THAT have been the main problem? =_= No newer games gave me even decent performance with this setup, except for Max Payne, which mysteriously ran beautifully at 1024x768 with 4x antialiasing O_o

How are Maxtor hard drives?  Oh, and I forgot, I can knock optical drive out of the overhead cost; I salvaged my DVD-9 Burner from my old system.