fist off all.... please change your name since RedUser is a real name forum (see thread above from Tim).
I'm using the GTX480 in the third slot as GUI, just as you can see in the post you quoted. I'm not really sure whether an other GPU than a GTX 480 would work as a GUI. As far as I know all GPUs should be the same. Also I think I read that DaVinci is using half of the GUI GPU also for processing and that a slower GPU would slow the whole system down. Don't take that for granted, since I never tried it.
@ Luigi: I installed your CUDA Generation update, but get an error quite often when I try to shut down the PC. Did you run into the same problem?
This week I'll finally have some time to work with DaVinci on a feature film. Any issues with long projects? I graded until now only commercials with it, so I'm happy for any tips concerning longer projects. The film was shot on Canon DSLRs, so I'm using 10bit DPX files.