Performance for now: Take Core 2 Duo (as its most compatible with todays games)

If u want futureproof the Q6600 comes in at about 100GBP and clocks to 3.4GHz very easily (even if you have to upgrade the CPU Cooler).

personally I would expect people building a new machine to go quadcore, else the machine will be outdated too quickly and you'll be stuck in a similar mscenario looking at another upgrade.

Just my bit. No whine aimed at me plz.