this is my spec
i5 750
4 GB
ATI 5770

how should I custom my graphic setting?

I had basically the same setup as you few days ago, except for the cpu. You should be fine running it in 1080p with customized medium / high, just don't expect solid 60fps.

If your cpu proves to be a bottleneck try turning off the SSAO & anti-aliasing. Also get rid of the blur effects, they just bother the gameplay in my opinion.

Thanks mate. Also, should I upgrade anything particular to boost the performance.

I can't really say should you upgrade your cpu since I'm not familiar with the Intel series, but I upgraded my 5770 to gtx560ti and I'm getting stable 40-60fps on ultra settings.

You could also check out some more ram, but contrary to the popular belief 4gigs is just fine for these days unless you run a gazillion processes at the same time.

High ^^ Both of your CPU and GPU are great.

no way, blur is gr8 shiet

I am playing in an Intel i7 960 w/ ati HD5770 and playing it in Ultra High at the highest res ( 1080) . Seems to do okay, but the frame rate does get a bit questionable at times, but not too bad, and the game remains completely playable.

My average frp is around 30-40 with this graphic setting.

Downscaling: None
Texture Memory: Large
Shadow Quality: Low
Shadowed Lights: Ultra
LOD Distance: Normal
Bloom: Enabled
Light Shafts: Enabled
Anti-Aliasing: Enabled
Blur: Enabled
DOF gameplay: Enabled
Vignette: Enabled
Wet Surfaces: Enabled
SSAO: Disabled
Motion Blur: Enabled
Cinematic DOF: Enabled
DOF Cutscene: Enabled
Dangling Objects Limit: Disabled
UberSampling: Disable
VSync: Disabled
Decals: High Spec