POWER: It depends on the specific PSU model you are using. Can you link it? As said earlier, as it's an Antec I assume you'll be fine up to a GTX 570. If it were a Corsair, you'd be fine up to a GTX 560Ti. This is merely because of the specifics of the two PSU models I have in mind and precisely why we need the exact model.
CPU: You've gotten a lot of misleading information on this. People have been saying that your CPU will match or overpower a graphics card and it's just not really getting to the point at hand, which looniam has given some advice on--most games will run great with a Q6600, but a couple will be CPU limited.
It entirely depends on the specific title, but the ones he listed are especially hard on CPUs. Some games are programmed such that a 4GHz Dual core in the world would struggle, but a 2.4GHz Quad would not. Others are the opposite and some, such as Source Engine games like Portal 2, could run great on a Pentium 4.
Because so many of the most popular games that you'd be able to play with a new graphics card are CPU intensive, I STRONGLY suggest you look into overclocking your Q6600. I've had very little trouble overclocking the one I've used to 3.0GHz on stock voltage. If you have a decent motherboard and cooling, you can go higher.
PLEASE post a new thread on overclocking your Q6600 and link it here. You'll want to pick up something like a Hyper 212+ for $15 when it goes on sale with a rebate. Just make sure it'll fit in your case first. You can transfer this cooler to a new computer in a year or two.
GRAPHICS: Once you start talking about high end graphics, there are diminishing returns on performance/dollar. For example, a GTX 580 provides about a 33% performance improvement on the 6870 for over double it's price:
http://www.anandtech.com/bench/GPU12/372
If you're using a 6670, I say just get a 6870 and be blown away by the performance. I personally own a GTX 570--but that's only because I got it for $200.