OK so i am selling my friend my old computer and build a new one. This is what i have so far -

MSI P45 diamond

Intel Core2Quad Q9550

Radon 4870 512 GDDR5

Antec Twelve Hundred

Antec 850W Power Supply

320gb WD HDD 16mb / just a temp one till i get my raptor

So i need your guys/girls help with 2 things

Ram - mine takes up to DDR3 2000 - How much and then reasonable speed

OS - Windows Vista or XP

Thanks for the help ^_^