A stock e5200 should be giving you an ave of 50ish fps on crysis warhead with a gtx 260 at 768res. It almost sounds to me like the processor isn't comming out of powersaving mode staying in 6x or 9x mode. I didn't think it could happen and I know you not ocing so its a minimal bios options but I might check disable any powersaving setting if there is one.
Just changed my power options from balanced to performance. Tried crysis again, same thing. I'm currently using 1680x1050 resolutions right now. I didn't try 1024x768 again yet.