On stock clocks, the 4870 is approximately 20% faster than the 4850 in most tests. If you overclock your 4850 a bit, you can close that gap to less than 10%, but of course you could OC a 4870 too.
The difference in power consumption is pretty negligible; 20 to 30 watts at load is the difference in usage. So assuming you've got a decent PSU, running the 4870 won't be a problem.
It's going to come down to how much you're willing to pay for that extra 20%. The price delta is over 50% still, so I'd have a hard time justifying it if I was on a budget. If you can swing $240-ish before rebates, then go for the 4870.


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