In the GPU2 guide, the last sentence under AMD/ATI at the top is "Also use the -forcegpu ati_r700 switch until 5xxx hardware is auto-detected in later FAH clients. Fixed in v6.30 and above." GPU3 is 6.31 for console and 6.32 for systray. The ONLY benefit for ATi users in GPU3 is an updated internal hardware list. It should run on the 5xxx series cards without extra flags.
The second one has quick links for the client downloads and pretty good installation guides for someone not already familiar with the GPU clients.
Did you use the drop in? There are people running SMP 6.30 without MPI after doing a clean install. Deino and MPICH are two methods for communication between the cores. A3 is thread-based, so the cores don't have to talk to eacbh other.