run the DX9 version if you have vista/DX10 card. really helps with frame rate. i have a 5.3 system rating and i'm running the DX9 version on max settings hitting an average of 50 FPS on the demo.
specs of my machine.
AMD Athlon 64 X2 3.1 Ghz
4GB DDR2 Ram
ATI Radeon HD 2900 XT 512 MB
Not true, here are my bench results from the demo that's been out for some time now.
Bench results.
DX version: DX10
OS: Vista Home Premium 64-bit
Resolution: 1920x1200
MSAA: None
Detail/Shadow/Texture options: Maximum
Test 1: AVG 101.25FPS
Test 2: AVG 77.30
Test 3: AVG 120.51
Test 4: AVG 67.31
AVG = average framerate
DX9 results
DX version: DX9
OS: Vista Home Premium 64-bit
Resolution: 1920x1200
MSAA: None
Detail/Shadow/Texture options: Maximum
1) 109.09
2) 76.52
3) 131.13
4) 78.68
Visually the DX10 mode has better lighting, much better shadows (no flicker either), and significantly better motion blur. Given the minimal impact on framerate (negligible to be quite honest), there's no reason to use the DX9 version if you have DX10 hardware and Vista; especially given that there is a rather noticeable difference in image quality between DX10 and 9.
Test system:
Core 2 Extreme X6800 @ 3.33GHz (10 multi x 333MHz FSB)
FSB 333MHz for effective quad data rate of 1333MHz
4GB PC2 8500 1066MHz RAM @ 1333MHz
nVidia GeForce 8800 GTS 512MB (GPU @ 800MHz, RAM at 2.2GHz)
Dell 24" LCD with native resolution of 1920x1200
PC Power & Cooling 1KW PSU
Auzentech Xplosion 7.1 Dolby Digital and DTS Connect sound card connected via TOS link to Yamaha 7.1 DDEX/DTSES receiver
Asetek Waterchill Xternal for CPU/Chipset
Thermalright HR-03 GT heatsink and 92mm fan for 8800 GTS
Tai-Chi case by Thermaltake