If you can get a 5830 for ~$40 less, or even lower, than 6870, than it's a good deal. If the price difference is below ~$40, you're better off with the 6870:
- if you plan to keep the cards after you finish mining, the 6870 is far better at gaming and also scales much better in crossfire
- if you decide to sell the card(s) when you finish mining, the 6870 will have better resale value. The 5830 was always considered a very poor gaming card, with 5770-level performance at above 5850-level power consumption. And with all the ex-mining 5830s on the market in a few months, don't expect to be getting a decent price for one.
- if you expect to still be mining for bitcents a year from now, the 6870 will have made up the price difference with lower power consumption (~30W)