I have a similar issue, the performance difference between Medium and High is enormous.
On Medium I have a VERY stable FPS of 55-60 (almost always stuck on 60, capped with v-sync, it rarely ever drops to 55), while on High it goes from 20 up to max 35, while most of the time it's around 25.
My hardware: AMD Phenom 2.6GHz Quadcore, 4GB RAM, ATI HD4870 1GB GDDR5.
I think it's because of my CPU though, I'm pretty sure my FPS will become alot higher and more stable on High if I just replace my CPU by a 3.0GHz or more.
So to the OP: I think the mainissue is your CPU, since the game most likely won't fully benifit from a Quadcore anyways, so even a 3.0GHz Dualcore would give you a way better performance (I'm not 100% sure though, just my guess, because games like this are usually very CPU intensive).
