What type of ram are you using DDR2 Or DDR3?
Well you're most likely gonna need a new PSU To run your new card. And I suggest a processor upgrade.
Newegg.com - OCZ ModXStream Pro 600W Modular High Performance Power Supply compatible with Intel Sandybridge Core i3 i5 i7 and AMD Phenom
Newegg.com - SAPPHIRE 100315L Radeon HD 6850 1GB 256-bit GDDR5 PCI Express 2.1 x16 HDCP Ready CrossFireX Support Video Card with Eyefinity
Newegg.com - AMD Phenom II X4 980 Black Edition Deneb 3.7GHz Socket AM3 125W Quad-Core Desktop Processor HDZ980FBK4DGM - Processors - Desktops