you can xfire 2 5830s. technically you can xfire a 5870 and a 5830, but it would be a waste, as the 5870 would perform like a 5830. So if you want to xfire a 5830, do it with another 5830, but id recommend the 460:
yeah, the 460 is generally faster, cooler, uses about the same wattage, i think a bit more wattage at idle. It also has much better performance with tesselation, and use of anitaliasing.
EDIT: Well I looked at the post about it in the forums here. Looks like the 460 is almost equal to a 5850 in many tests. I'd definitely be looking at a 460 for the $200 price. Assuming they're not all selling out that is.
from what ive seen, the 1 gig version goes for about 85-95% of the performance of a 5850, and the 768 mb version does about 10-20% more than the 5830.
My mind is blown by the 460 lol. A $200 card performing like a $300 card. I read the review at Anandtech, and OCed, they perform almost equal to a 5870... and in SLI vs CF they even beat CF 5870s by about 1fps. Granted, that is OC vs stock, however from a $200 card that's so amazing. The 5850 is touted as being a good buy because it can OC to slightly better than a 5870 stock, but for $100 less I'd get the 460 right now. 1GB verion, of course.