bios P32 has the best overall 45NM performance and stability. Some guys like P31 better so its really up to you. The microcode update in p32 is worth it when it comes to new silicone.
I dont reinstall windows when switching out procs. The one thing you may want to do is is clear the Cmos after you switch out the processor. When upgrading from my E6600 to a Q6600 my board flipped out a bit but came back after a CMOS clear.