my comp Lags, want to make changes.

my comps specs-

Dell DIM2400
Intel Celeron (R) Prc. 2.40Ghz
2.30 Ghz, 512MB RAM
70.5 Gig HD 5,400 RPM
Intel Extreme GraphicsC (R)(doesnt say MB, but Enough to run World of Warcraft)

thats basically it. I Lag so bad on WoW and even some on WC3. I lag somewhat on Unreal Tournament. I want to know what changes i need to make to pass the lag or or if i should just save up for a barebones and put "the best" if what it can get into it.

I have a friend who has a DIM4800 (same as 2400 {mine} but a significantly smaller tower and it has P4). His comp used to lag until he put an extra 128 into his RAM. His comp doesnt even think about lagging. I highly doubt if i did tht it would stop my lag.

My comp on the internet is a one click load comp. Theres no wait when i click a link, but for games.. yea right. plz help. thnks.