7800GTs are fairly quiet if the fan speed control works. On many cards, it does not. Instead of ~25% speed in 2D and ~45% in 3D, you get 100% all the time.
Manufacturers either don't know or don't care what the cause is. Cards with this problem can be moved into completely different PCs and have the same problem, so it is unlikely to be a config issue. A clean installation of Windows is claimed to fix the problem, but I do not know of any cases where it actually did so. RivaTuner and any other similar programs cannot control the fan speed on cards with this problem. Although it seems most common with XFX and BFG, it has been reported with Leadtek and eVGA cards, probably with other makes too. However, it's only with some models in the range. For example, the UDF7 revision of the XFX 7800GT Extreme Edition does not have the fan speed problem. The UDE7 and UDER revisions (which, despite the name, are more recent than UDF7) do have this problem (stability issues at stock speeds are also reported with UDE7, which is probably why it was quickly replaced by UDER). Maybe the fan speed control will work on the card you buy. Maybe it won't.
7800GTs are the Dirty Harry of graphics cards: Are you feeling lucky?
If you don't care about having a guarantee on a £250 card that you already know has a serious flaw, you can replace the cooler with a specialist one from Zalman, Arctic Cooling or whoever.