Having tested over a dozen cards from the 4xxx, 5xxx, 6xxx and 7xxxx Radeon HD range my current favourites are:-
the passively cooled Sapphire HD5670, the half-height, single slot Sapphire HD6570 and the passively cooled Sapphire HD7750. These cards deliver a good balance between power, performance and noise level (i.e. no noise with the two passively cooled cards). In addition the HD5670 and HD6570 cards are both compatible with the most recent Linux kernel created by Darren Stevens, which is a nice little bonus. The two passively cooled cards take up more space but the HD5670, due to its form factor, does not use a second slot.
The 6850 is also a nice single slot card but does include an additional cooling fan.
TrevorD
For more info check out my blog
http://blog.a-eon.biz/blog/