Need help for a very budget build <400

Hi, my brother needs a newer system, his computer is getting way too old now, and he doesn't want to spend too much, since he mainly plays console games... I recently built my own too, but all I read up on was the latest hardware, so I'm a bit lost when it comes to older hardware. I will do my own research today, but I wanted to get some quick help too so I can save time in deciding what to buy for him. You guys don't need to link the websites if it takes too much time, I just need some parts recommendations, then I can go look for it on my own.

This is his old computer, for reference:
HP a510n
We added an extra 512MB ram for total 1GB and added a video card (nvidia 7600GS)

APPROXIMATE PURCHASE DATE: Now

BUDGET RANGE: ~400

SYSTEM USAGE FROM MOST TO LEAST IMPORTANT: Adobe Photoshop/Illustrator, and World of Warcraft @ 1680 x 1050 (the only computer game he plays)

PARTS NOT REQUIRED: keyboard, mouse, monitor, speakers, OS

PREFERRED WEBSITE(S) FOR PARTS: tigerdirect, newegg, zzf, or any California tax exempt

COUNTRY OF ORIGIN: United States

PARTS PREFERENCES: no preference. best bang for the buck, price/performance ratio

OVERCLOCKING: Yes, to squeeze best performance

CROSSFIRE: No

MONITOR RESOLUTION: 1680x1050

ADDITIONAL COMMENTS: Is a $400 computer possible? If it needs be, we can reuse HP's power supply or case (micro atx). We can also reuse the DVD writer. It's not too imporant. In that case, all we would need is motherboard, cpu, ram, video, hard drive.