You should be able to run WoW on Vista with 1GB RAM although your load times will be a bit lengthy. Not like BF2 lengthy or anything so it probably won't matter much. I'm running Vista RC2 on 1GB RAM right now and I with Aero and Sidebar, and I have anywhere from 300-400MB free. My video card is builtin so I actually have 896MB RAM - if you have ur entire 1GB intact you should be alright with WoW.
Not so sure about Halo 2 though... it's graphics won't be spectacular but it IS a DX10 game. I suppose it will run decently but you'll be a bit disappointed at the load times.
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