The PC3200 kind, that is first and foremost stable. I'm tired of bad ram making my computer crash >_< I had 2x256 but now I can only run with 1x256, doesn't matter which one....:'(
Edit: Whats this about DDR2, can I use that in my p4e mobo?
My suggestion is, whatever brand you get, make sure they have a very long warranty period. I recently have a broken PC133 DRAM on an old computer (it can't pass Memtest86 and sometimes the computer won't boot), and the manufacturer have a life-time warranty for the DRAM module. It took seven days to get a replacement, but it's free.
And it's probably better to test any new DRAM module with Memtest86.