Hey everyone if I upgrade my socket AMD socket 939 3500+ to a duo core 4200+ (its a 939 processor of course, so I'm only switching out processors, not mobos or anything else.), do I have to reformat my hdd or can I just switch them out and everything is good to go?

You wouldn't need to do anything with the Harddrive. Possible WinXP could balk at the change due to thinking this was a different computer but I don't think that would be an issue. I would not recommend spending the money for that upgrade. That isn't enough of a speed increase to justify the expense. Why do you feel the need to do this? Possible that other upgrades may be more benificial.

its a massive preformance increas, do not look at raw core speed, the 3500 is single core vs the 4200 is dual core. The diffrence in the speed of the machine is noticeable imediatly, its a good upgrade to make, and the first one i ever recomend.

"its a massive preformance increas" I don't see how it could be "massive". The 3500+ runs at 2.2GHz & the X2 4200+ runs at 2.2GHz, & they both have 512k L2 cache. Obviously the X2 will be better at certain applications that can make use of the dual cores, but other than that, it's basically a wash.