Overall, since I'd think that most people spending $170US+ on a video card would want anti-aliasing, that the 4850 would be the clear winner; the 9800GTX+, let alone the 9800GTX, can't really compete too well with the 4850 when AA is running, especially at higher resolutions.
However, as noted, the default cooler for the 9800GTX, being a dual-slot one with a vent out the back, is superior, and it seems that, perhaps in part due to said cooler, the card tends to overclock a bit better. However, as noted, there are versions of the 4850 with dual-slot coolers, as well as compatible aftermarket coolers available for the card.