Then I would go for the 4850 it about ties the 9800 GTX oc'd and kills it with AA enabled. Where the 4850 will really shine in this case is crossfire since it will scale better and in future titles, the 4850 will perform better than the 9800 GTX. Make sure to get an X38 or X48 motherboard since the 4850 takes a large hit in crossfire at anything below x16/x16. I would also recommend waiting a week or 2 to see if some custom coolers come out for these cards, I hear the 1GB version has a nice double slot cooler.
I would go with the 4850s from the reviews I have seen.
They are generally as fast or faster depending on the tests.
And the tests they win they win by a decent margin and when the lose, they lose by a small amount.
Since the 4850 is a new generation card, their drivers are newer and games have not been fully patched to address possible performance issues with the new cards. As a result, performance is likely to increase more over time for teh 4850s than the 9800s
With NVIDIA's chipsets for Intel CPUs in the state that they are (don't look very good in everyday usage) I would pick a X48 and 2x HD4850.
I would however wait until the HD4870 comes out.
That being said the 9800GTX is a good overclocker and the 9800GTX+ will very likely be an even better one.
Good point, and one that many people seem determined to overlook. Data corruption issues are still a reality with many of the newer nVidia chipsets when overclocking, and yet people persist in buying them.