Please help

Hello,
i want to buy graphics card for my system.
configuration of my system is:
1. phenom X4 9550
2. MB Asus M3N HD/HDMI
3 RAM 4GB 800 Mhz Dual Chanel
4.320 GB Hard Disk 5400 RPM Hitachi
5. Monitor Dell 19"
6. Asus xonar essence STX sound card
7.dvd Drive
8. PSU Cooler Master Extreme power plus 650 W (36 Amp On 12 V Rail)
9. Altec lansing MX 5021
which one will be better HD 6850 or 560 gtx.can my PSU drive them comfortably?
(m not going to overclock anything)
is my CPU/ RAM/ HD going to bottleneck The GPU i mentioned?
in that case what is suitable upgrade for prossesor or ram?
or what GPU i should go for to get max out of current configuration?

please help me out and oblige

Thanks and regards

amol ambulkar