which is backing me down of these three:
Core 2 Duo E7200 2.53 stock
hd 3870 gddr4 256bit
the resolution i use is 1024 x 768,but most of the people say that hd 3870 is fine for that resolution,my cpu is backing me down,is it right?because at that low resolution games become more cpu intensive instead of gpu instensive.
Yes,it's right.Lower resolutions benefit mostly from CPU and HD 3870 can handle all games fine on your resolution.
If you are going to upgrade,upgrade to 4GB RAM first and buy a new monitor before upgrading your VGA.
Well,like i mentioned,getting 4GB RAM will give you a good boost in performance.
A faster CPU will also help but your resolution is also low,so upgrading your monitor lets your card run at its full potential
Really the thing holding you back is more a combination of everything --- It is a fairly balanced machine but it is a low - mid end setup -- most newer games will run better on a system with better than a 3.0 ghz. speed so the lower 2.5Ghz. speed of your processor slows down the throughput - having only 2GB. of RAM when the current OSes are using around 1 GB. just to load themselves adds to the problem -- then having an older GPU (they are on the 6xxx series now so a 3xxx series is 3 generations older and things have changed a lot on performance in 3 generations !)
So while no specific part is really holding you back that system is not going to get more than low to mid level performance as it is a system made with mid level at best parts and improving that will mean improving all of the components not just a single one since any one part that is upgraded may add a bit of performance but then will be held back by the other components.