I've been using a Sapphire X1900XT for the past few days, and it's a tad louder than my old 7800GTX 256mb card (I don't count the noise at boot -- it's only there for about 3 seconds or less). My Thermaltake Xaser III case is large and heavy (all steel), but the 4x80mm fans I have running pretty much drown out the X1900XT's fan. I'll probably add 2 more fans this week, which should all but smother the X1900XT.
But as loud as the my PC is (all stock cooling), it's still not nearly as annoying as my Xbox 360....
I hear that the quietest and most efficient fan is the Arctic Cooler X2 (or X1 for the 7800/7900 nVidia cards). But the fan does not blow the hot air out of the case, so if your case is small or cramped, this probably isn't a good option. I read one post on this forum that said the card still ran in the mid-80's.
If you're worried about the warranty, then wait a few months to at least make sure you don't have a defective card. If you're lucky, some manufacturers might even honor the warranty with aftermarket coolers, though these are in a small minority.
Sapphire also has designed an X1900XTX that's liquid-cooled, but I'm not sure if it's been released yet.