Go here to check the spec diffs:
http://www.gpureview.com/I agree that the 5830 is a better buy than the 5850. I personally just don't trust the nVidia drivers and way-high power consumption. But then there are those here that don't trust the ATI drivers and don't care about power usage. Both the nVidias & ATIs are good cards, with problems of their own. My experience and observation has been that ATIs have less, hence my swing away from nVidia to ATI. And I still think that the ATIs represent better value for money.
Using the website I gave you, you can do comparisons between the 5830 & the nVidia 460, 470 & 480s. Presumably you'll be purchasing thru TM or PriceSpy (the place where people go to purchase from those who sell for next to no margin and ruin the computer industry

), so you'll be able to see how much dosh each of the cards is vs performance and power usage.