you want max settings... and you want it to run smoothly....you need to upgrade the CPU and video card. but yeah, need to know you PSU make/model, motherboard make/model and monitor resolution. For max settings with 1920x1080 resolution a gtx460 or better and phenomII x4 would be a good start.
would agree with the people above who said you need to upgrade your cpu, but at least try to OC it a bit, and If you have an ample power supply(which you need to let us know about) then I would say go for something in the ballpark of a gtx 460 or better. Your cpu will still be holding you back a bit... but you can still have a decent setup with a video card upgrade as previously mentioned but what kind of psu do you have? make and model?