My first initial thought when I saw the subject line was, "Well that depends...do you want to keep him as your friend?" lol.
If yes, then install XP Pro.
If no, then install ME. lol.
I'm running XP Pro with an older hd...it runs at 5400rpm and the games play fine for me. Granted...I've yet to compare it to a hd with 7200rpm, so who knows I might be missing out. Perhaps someone else can shed more light?
Either way...XP is the way to go IMO.