If you want to play a game like HL2 decently, you'll want to upgrade just about everything... ram, cpu and video card. Upgrading the ram would be a first priority IMO, followed by the cpu. Although that in itself probably isn't enough to run HL2 at a decent rate, it'll give your computer good flexibility to do everything else. After you're tired of playing HL2 on the smaller resolution with everything turned down, get yourself a nice graphics card (I used to be a Nvidia Geforce fanboy, but converted to ATI Radeon after I bought a 9800pro awhile back
). Thats more or less a new computer, minus the motherboard and a couple of other components. So you might want to look into buying a whole new system... like quinn said before.
-Richer