best pci
The 5600 is better than either that you listed. The 5600 should have 325mhz core clock, 550mhz memory, and 4 pixel pipelines. The 5500 should have 250mhz core clock, 400mhz memory, and 2 pixel pipelines. The Bfg 5500 is overclocked to 290mhz core but it looks like the memory is only 350mhz, so it's a little faster than a standard 5500 for core speed, but slower in memory speed. The 5600 has a 35mhz core speed advantage over the 5500bfg, and a 200mhz advantage in memory speed. The 5600 also has 2 more pixel pipes which let it process more data at a time. The 5600 is better, but it is more expensive.
cliff notes:
5600 325mhz core, 550mhz memory, 4 pixel pipes.
5500 250mhz core, 400mhz memory, 2 pixel pipes.
5500bfg 290mhz core, 350mhz memory, 2 pixel pipes.