What RAM do you have? If it's DDR, then I think you may struggle to get a decent DDR CPU-mobo combo, unless you don't mind getting something off eBay.
But for your budget you could also get decent DDR2 RAM, so shouldn't be a problem. Check out the mobo's dimensions to see if it'll fit in the case.
Total = £248 for the following:
- Intel quad-core Q6600 £156 click here
- MSI P35 Neo-F £57 click here
- OCZ 2Gb (2x1GB) DDR2 £35 click here
Of course, you could go for a dual-core E6750 click here (or the newer E8300/E8400) instead & save a bit.
OR, you could just go for a new system with similar spec to the above & a better graphics card.
I'm surprised a mere 8600GT can handle Crysis even on medium settings. Don't own the game though. G