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Don, Q6600 still good if not overclocked? I don't dare fuck with overclocking things but that's the processor I'm eyeing for my upgrades atm; and what's a good model mobo to go with it? I've only really used AMD setups before so intel models and socket names are newish territory :P
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XP = win, Vista = fail; 4gb = 32bit as Don said, over 4gb, go 64 but beware driver issues, XP64bit was the "first" 64bit windows so it's got some shortcomings/compatability issues, Vista64's more prepared for it but it's still Vista, so ew. :P

Just cuz you CAN SLI doesn't mean you SHOULD ;p Get an 8800GT imo, the 9x series doesn't run as well for some technical reason or another, and gives no performance increase over an 8800GT which is overkill anyways.