I recently upgraded my rig and got some great performance boosts.
Old system:
Mobo: Asus P5Q
CPU: C2D E8500 @ 3.8Ghz
Ram: Corsair Dominator 4GB DDR2 @ 1066Mhz
HDD: 250Gig and 500Gig 7200 RPM Barracudas
GPU: Geforce GTX 275 896MB
Sound: X-Fi Titanium Fatality
PSU: Ultra 750 Watt
DVD-RW: Sony
New System:
Mobo: Asus P7P55D-E
CPU: Core i5 750 @ 3.4Ghz
Ram: G.Skill 4GB DDR3 @ 1600Mhz
HDD: Kingston 64Gig SSD for OS and select games
GPU: Geforce GTX 275 896MB
Sound: X-Fi Titanium Fatality
PSU: Ultra 750 Watt
DVD-RW: Sony
At 1680 X 1050, went from medium - high settings and 1500 draw distance running at 22fps in city areas to high everything (except terrain and no AA) and 3000 draw distance running over 30+ FPS at all times.40-50 in outdoors areas. Load times are also astounding with the SSD and there's very little stutter if I spin around in a new area.