Regarding future-proofing and the value of quads, - in short, go quad if you do any serious media production work, or if you want your system to last more than a couple of years. Although I would generally recommend quads, all things being equal, the additional cost has to be justified when working to a small budget.
Given you're a gamer, you're not against the idea of overclocking, you might be happy replacing within a few years, and that money is a real factor in your costs, then a mid-range dual core could be a good gamble
Remember folks - that possible saving of nearly £40 between a Q6600 and an E8200 is significant, it's 10% of the total system cost!
You're also right to not replace your graphics card yet, you'll get much better value for money in terms of the increase in power in a few months.
I'll let others like StaffsMike roll out their generic £500 system details for the full component list now, since they have got a system designed perfectly
We probably should get a "£500 gaming computer" thread stickied,
tbh