If price is an issue, the 6770 is exactly the same as the 5770, which can be found for $110, and with rebates, $90. Here's an ASUS one: http://www.newegg.com/Product/Product.aspx?Item=N82E16814121363
AMD simply re-branded the 5750 and 5770 as the 6750 and 6770, but the new models tend to cost a little bit more. The 6790 is much like the 5830 - it's a further disabled 6870 than the 6850. Most people think it's a Juniper-based GPU (5770) with a 256-bit bus, but it's a heavily-crippled Barts GPU (6870).