Don't get the 5830, it's a dud of a card...
About overclocking the 5770: mine does 930/1300 stable, according to GPU-Z my memory bandwidth is 86.4Gb/s, Pixel Fillrate of 15 Gigapixels, Texture fillrate of 37.6 Gigatexels.
For all it's worth, overclocking gave me a massive boost in FPS with DiRT 2. at stock settings i get a min fps of 28, when overclocked i get a min of 38, which is a nice improvement.
I would seriously wait until the GTX 470 and GTX 480 are released, and then see how the prices go on the Radeons. You might be able to afford a 5850.