Dude, if you got $300 to spent to get both PSU and GPU, I'd say get a capable PSU first and then the rest of the money go for any GPU it could spent.
And, I'd say a Seasonic 450watt is capable enough for any GPU you can think of as long as you don't double them in the future. I believe it should cost you at hovering somewhat lower than $100ish. You can spend the rest of the money to 6870 or any card on that price level and would still get a great performance card.
TBH, a $200+ cards are somewhat a degree your CPU could handle for its best performance. Since Q6600 is equivalent to a bit slower than a dual cores SB in terms of raw power. Any higher than that, you're kinda wasting your money. Sure, a $300ish GPU would still help to raise the performance bar to another degree, but not as it actually should. Plus, if you are still mortals like most of us, you won't really know the real difference of $200 cards and $300 cards unless you do a benchmark comparison, or using an uber large display settings.
And I won't suggest you to overclocking it either, the risk is too damn high. The only viable option to crank Q6600 is by overvolting, sounds like a dangerous thing to do for me if you consider the lifespan for your chip. I'd say just leave it be, it should deliver enough performance on stock speed for any -$300 cards anyway.
Oh, and I really can't emphasize enough the need for more memory. A decent gaming experience can only be attained at at least 4GB Dual Channel Memory regardless what kind it is, be it DDR3 or DDR2. Even a Dual Channel 2GB won't cope the burden. The OS and systems drivers and all the thing that makes your PC works alone should tax you something like 700Mb-1Gb RAM. You'll get the picture.