Help me to upgrade my pc

Hello,

I plan to upgrade my pc and iam a graphic designer and internet user, i also willing to play the latest pc games. I already have intel core 2 duo 2.4 ghz processor, 500gb hard disk and 19" lcd monitor. So please suggest me a best option for buying intel motherboard, ram and 1gb nvidia graphic card. Also which operating system is best for gaming ( windows xp, vista, windows 7), also plz tell me more about ram for good speed