Need Computer Buying Advice for Playing Runescape & Warcraft?

My son has a Windows ME Desktop computer that freezes playing strategy games. Current is P3 933mhz, 256 Ram, 32 mb ATI Radeon graphics and 60g hard drive. I'm computer literate & can configure computers for business, but gaming is different. It isn't for hard core gaming & would be nice if it was Vista ready, but don't want to break the bank....he is only 14 and who knows what he'll be downloading, etc. I'm currently looking at a Dell E521, Athlon 64 X2 Dual Core 3800 processor, MCE, 2 gig Ram, 256 mb ATI Radeon X1300 Pro graphics card, 250 gig hard drive. About 785 after tax. This is for what I THINK I need, but am hoping someone can shed light on exactly how much mem & graphics I need and lead me to something for less. From the limited amount I know about gaming, Runescape isn't 3-D so do I need this much??? Would love to be closer to $500. Have also looked at HP Media Center systems? But many are integrated graphics. So confusing! Oh - don't need a monitor. Thanks!