Nickg, you'll be happy. A 8800GT is a nice card, but the 5830 is a big step up. It's a nice stepping stone into DX11. When you think about it, why buy the expensive 5850/70/Fermi when we don't even have enough DX11 games to judge them by yet?
It was similar to the 6800 series from Nvidia and Shader Model 3 (SM3); people bought expensive Ultras for next gen support, but they couldn't run mainstream SM3 games well once they started to become the norm. We had to wait for unified shaders.
I intend to use the 5830 as a transition card, then get the next gen refresh once we know what's needed for future DX11 games. If they become tessellation heavy, the next gen DX11 will be where it's at. In the mean time, we get to put our foot in the water!
Is it just me, or is it at some places instead of the 5830 coming down in price, the 5850's have gone up in price? High prices seem to be the norm and don't look like coming down anytime soon. I expect the 5830's to sell well once they hit the sub £150 bracket, if any exist by then (if 5870 yields improve, this card may disappear or be replaced with a different model).