I had the same PC parts except 512MB RAM. I could barely run HL2 at all. It was a slide show. It ran, but very poorly. When I got 1GB RAM it ran smooth as freaking silk, man!
AH runs better, smoother. Less swapping. Programs load faster (Photoshop barely blinks the screen that loads filters, etc, before it's ready to use), and everything is faster, and generally happier. I just wish I had 2 GB in dual-channel. That would rock.