Why don't you keep your Q6600. Find a decent board with overclocking capabilities. Second hand for £30. Get a third party cooler. £20. Overclock it to anything past 3.2Ghz.
Depending on your ram. Get some second hand PC2-6400 stuff for £30 (4GB)
Keep your HDD, if it's sata... it'll be fine.
Then all you need to splash out on is a fancy new GPU. Get the best you can afford. £200 left.... so a 6950 would fit in the budget nicely. That will be a lot better for BF3 than spending £300 on a whole new range of kit.
Upgrade the rest when you can afford it. The GPU will be fine to swap into a new build obviously. Your PSU is obviously fine also.
If your current mobo is upto the job of overclocking... then quids in.