Heya,
$119 for an HD4850 at NewEgg.
$500 for an HD4870x2 (don't know where that $379 number above came from for the HD4870x2, it's $500).
Even Crossfired, the HD4850's are about half the cost of an HD4870x2. Performance wise, both these setups will be absolute overkill for most games you play. Very few games will actually be able to even remotely tax these two setups, other than the latest crunchers (like Crysis) though that's not due to them being too graphically complex, but actually do to really piss poor coding (eg, Crysis).
For the money, I'd go with the HD4850 in crossfire. Or a single HD4870. The HD4870x2 is the fastest 'single' card on the markget, but it's also the most expensive. Is the overkill worth the $500? I don't think so.
Cheers,