I've got a 560ti. Runs everything at mid-to-high settings, generally with shadows and post processing being the first things to get turned down. You have to fiddle with the settings to get a decent balance between frame rate and looks, but it's not a bad card for just $200. I'll be upgrading to a 600 series GPU once I get my new computer, though.
I recommend aiming high on graphics cards, too. They're one of the core components that will make games run fast and look nice, so if you can afford it, going into the $300 or even $400 range is an investment that'll keep you for a good while before you even have to think about turning the graphics down on new games. You may want to wait to see what the new consoles are going to be packing, though: Most near future AAA games are going to be based around their hardware, so if you're uncertain it'll be a good start to know what ranges you're looking at.
Also, something to know about GPUs: The number of GB on the things don't really matter as much any more, and 2 GB is plenty even for the meatiest (Affordable) GPUs, so don't pay extra for anything that has more than that (At least, until we get around to newer models that do need it). It's basically RAM on the GPU, and if the GPU's fast, it'll be pumping data out quickly enough where having more gigs won't increase performance in the slightest. What you're looking at is how many bits it has and how many MHz it runs. Higher is better, naturally, but go for higher bits before MHz.
If you're planning on upgrading with dual GPUs in the future, remember both GPUs have to be in the same series - So a 500 series won't hook up with a 600 series. If you're thinking about doing it right now, remember that not all games support it, and you'd be better off dropping $500 on one GPU rather than $400 on two.
Another thing that's good to know, PCI slots on your motherboard don't always need to be x16, either. 99% of the time with new cards, the jump from x8 to x16 is marginal. The big thing you want to look at is whether it's 3.0 or 2.0. See, a 1.0 x16 is as good as a 2.0 x8. It's multiplicative, so if you get a 3.0 x8 PCI slot, you'll be running just fine. You might get a 1% improvement if you jump to a 3.0 x16 from 3.0 x8, and that's only with the best cards on the market. Also, they're basically all compatible with each other, so you don't need to go and try to match up PCI slots with your GPU, and you don't have to worry about compatibility if your GPU is still around when/if they come out with 4.0 PCI slots. This all mostly comes into effect when using dual GPUs, since some motherboards turn their x16 slots into x8, but don't worry, it won't be a big deal.