OK, I've decided not to upgrade the Graphics Card, for now. If I need to get one later I will, but I'm definitely keeping the 8600GT for now.
This means that my budget is now around $500 for just the CPU, Motherboard, RAM and PSU.
There does seem to be a good argument for going with a Phenom but gong on all your advice, I'll stick with an Intel.
However, now I'm tossing up between a E8500 and the Q6600. I can get both for around the same price. From my understanding, the E8500 is better at the moment (with its 45nm and superior speed) but the Q6600's extra 2 cores will be utilised better in the future, thus increasing the lifespan of the chip.
Would you recommend staying with the Q6600?