Upgrading CPU, same HD possible?

Hello,
I am looking into upgrading my CPU and motherboard(5-6 years old).
Current CPU: AMD Athlon 64x2 Socket AM2
Motherboard: ASUS M2N-SLI wireless edition
Pretty old stuff. Something in my computer died yesterday(either mobo or vid card) and so I figured I might as well upgrade my CPU and mobo. My vid cards/RAM/Hard drive are still intact, so if I get a new CPU, would I need to reformat my HD? Using Windows XP Professional SP3.
For instance:
I switch from current AMD dual-core to AMD quad-core and compatible mobo, would I need to format hard drive or would the transition be smooth and windows would be able to do everything?
Another instance:
I switch from current AMD dual-core to Intel processor(one of those i-series things) and Intel compatible mobo, would I need to format hard drive or would windows do everything for me?
Don't want to start a war, but any suggestions as to which one I should switch to(mind you, if one of these doesn't require me to format hard drive, that wins hands down)? I play computer games pretty often and with my current setup, its impossible to play on medium/high(not max) settings. I've gotten used to playing on low settings, but I would like to experience no lag with good graphics =D.
Current video cards: 2 x PNY GeForce 8600 GTS (maybe its video cards that are holding me back?)
RAM: 2 x 2gigs Patriot DDR2 (800 I think).