The Radeon 5770 for $159 is nice because you get DX11 and very, very low power consumption (saves you a bit of cash in the long term). It is undoubtedly slower than the GeForce GTX 260 or Radeon 4870, however. The best performing card you can get for that price range is the Radeon 4890, but it is not DX11 and has very high power consumption.
Here's the general performance ranking of that range:
1. Radeon 4890
2. GTX 260 / Radeon 4870
3. Radeon 5770
Power consumption ranking
1. Radeon 5770
2. GTX 260
3. Radeon 4890 / 4870
I'd get this:
http://www.newegg.com/Product/...x?Item=N82E16814127430
$30 more than the cheapest 4870 1GB, but it's slightly faster thanks to a factory Over Clock. You might not make back $30 in power consumption ever, but it does indeed use less than a 4870 or 4890. It's not quite 4890 performance, but it's reasonably close thanks to its OC. It costs less than a 4890, and uses less power. You also get Physx support if you wanna get one of those games.
DX11 support if truly the only wildcard here. If you want it, then the 5770, while good for 1680x1050, is still a bit slower than the other cards you could get. Ponying up to the Radeon 5850 gives you better performance than anything else here, but costs $259.
Here are some tests with the Radeon 5770 compared in Call of Duty WaW (more demanding than CoD4, less than Modern Warfare 2). You wanna pay attention to GTX 260 and Radeon 4890 whenever they are there. Only look at the 1680x1050 results, of course.
http://www.techspot.com/review...eon-hd-5770/page4.html
http://www.hardwarecanucks.com...gb-gddr5-review-9.html
http://www.hexus.net/content/i...php?item=20670&page=10
http://www.bit-tech.net/hardwa...adeon-hd-5770-review/9
http://www.pcgameshardware.com...-Range/Reviews/?page=3
GTX 260 seems very nice. Modern Warfare 2 is only DX9 anyway.