At the most, I'd say the video card. It's enough to play BF3, at medium and high settings with no AA and probably at 720p.
But a HD 5770/6770 or a HD 6790 seems like the highest you can go with, since you've got a stock E7400.
On NVIDIA's side, a GTS450 (equivalent to a GTS250, but not equal to your OC'd one) or a GTX550Ti (which I wouldn't recommend? is the most you can go for.
To get more, I'd say you change your setup entirely. It's looking a bit old.