6850 seems a good choice: low power draw, single PCIe power connector, solid bang-for-buck at 1080p. If it is a reference model, it should have the "blower" cooler (which will help in a poorly-ventilated case), but the non-reference cards do a good job too:
http://www.anandtech.com/show/4002/amd-radeon-hd-6850-overclocking-roundup-asus-xfx-msi
http://www.anandtech.com/show/4002/amd-radeon-hd-6850-overclocking-roundup-asus-xfx-msi