eVGA is one of the more popular ones. They have a good warranty that covers overclocking and they have their step-up program which allows you to upgrade the card for the difference within 90 days of your purchase. I have also heard good things about BFG and XFX.
I would like to throw the Asus EN8800GTS 320 into the fray, having had the opportunity to fiddle with one for a bit I can attest as to it's overclocking potential, it's not the cheapest of the standard clocked ones however, but you can get it bundled with G.R.A.W and a full copy of 3D '06.
I have an eVGA "Superclocked" model, and highly recommend it. Good price, good stock performance, and overclocks like nobody's business (have had the thing running semi-stably at 675/1150, although not good for long-term at that speed. Gets a really nice three hours or so before throwing out a STOP error, though).
I think generally the way it works is that XFX, BFG and EVGA are the most expensive, but also come with more (factory OC, bundled games/software etc) and have better warranties that last longer and/or cover OC'ing (although so long as you don't tell the other companies they aren't going to know it was running at 200 degrees c owing to your OC) but I have actually only ever had experience with XFX, whom I give a thumbs up but never really had to use the warranty so (oh and I may be completely wrong, but I don't think I am)