Push it as hard as you can, my previous E6600 /5770 rig was still showing performance gains at 3.2GHz (stock 2.4).
I'd suggest a bit more power, though, 500W as a minimum and do NOT forget to add a better cooler to the CPU or you could damage it through overheating.
Crysis Warhead needs a massive machine to run it well, even at 1280x1024.
Try a few tweaks to improve the performance: Shadows and other lighting effects can soak up a great deal of processing time and turn the physics down a notch as well.
Do not worry about the PCI-E version, the HD5770 is nowhere near fast enough for it to make a difference, PCI-E 1.1 will barely slow a GTX295 for example.
Before spashing out on a new CPU make very certain the motherboard will support it, some will, some will not and some need a BIOS flash in order to support a different CPU.
Rather that taking that route you might want to consider overclocking, given some decent cooling the E5200 is quite capable of running much faster, but results will vary and depend as much on motherboard/RAM as the CPU itself.
Just remember: Overclocking is like a box of chocolates, you never know what you're going to get;)
Do some research first, you CAN destroy the CPU and do other damage when overclocking.
And as a final warning, overclocking can be addictive
Have fun, mate!