i would agree with cyle b. at the same time, quad duo id better. it is a quad (4) core processor with a major overclock speed which i do not know off the top of my head. core duo is a (2) core processor. i would go with the quad, but, personally, i don't have 250 bucks plus to hand over to pay for the thing. secondly, you will want to have a lot of ram (for the game you have listed, i think the minimal is like 1 gig). i would go with 2 gig's min. as for polaying multi-player online, i do not know how you are going to connect to the internet, but i suggest having a faster speed lan connection or wi-fi connection. whichever you prefer. i would go with the integrated LAN becaus it is a lot cheaper. things like power supply, if you are building this yourself, you will want to have enough to be able to power everything in your computer, and, just a little extra juice to be safe. typically, the standard desktop now a days runs about 250 watts, but for you, i think like 500 or maybe even 750 watts. call of duty 4 is compatible with xp i do believe (this is the better operating system) so go with xp pro SP2. this should suite all your needs and save you some here and there. as for vista, all i got to say is they got some bugs to work out. for this system i am describing, it will probably cost anywhere from 1500 bucks already built for you, on up. hope this helps.