He can OC the PhII x4 955 BE without any voltage adjustments (probably around 3.8-4.0 gHz at the most). All he has to do is change the multiplier setting in the BIOS and he should be fine. If he requires a voltage adjustment, than he'd be using more power to get the desired OC that he wants. A quality 400w or better PSU is all that he'd need, unless he wants to add more parts later than a 500w or better PSU should be required. I like Antec/Corsair/Seasonic/PCP&C PSU's for all of my builds.
Windows 7 64bit is a better option than the 32bit version, IMHO. You can use all 4 gb's of RAM and also upgrade to more RAM later. Currently Newegg is having 8 gb RAM kits for around $20-$40, so that would be good option to have, if your mobo supports it.