100 pounds is the maximum price you should pay for one. I have the 5750 which is a slightly crippled version of the 5770.
Originally I had some problems with Linux, the screen would go black (out-of range screen mode) mid install, but not anymore.
They are quite fast and should handle medium graphics settings fine. I only noticed a slow down on Civ 5 (takes about 2secs to draw the screen) with everything on max, but possibly because there was not enough memory; 1GB graphics, 2GB system.
No problems with any movies.
I believe the 6000 series run cooler.
Check this table to compare models. Pay attention to the memory bit width and the number of texture units etc.:
http://en.wikipedia.org/wiki/Comparison_of_AMD_graphics_processing_units
