For question #1, you should probably stick with the suggested cards from Chris (in the other thread). With that said, other OEM cards that fit and are the same series/specs should work, but you would have to test them out first since this they did not originally ship with this system model. For question #2 the PSU model number is: T7X04, and this is correct on the amount of watts that have been spec'd for the system.    
Hi, soon I will receive new Vostro 3910 (with standard PSU 180W). Thank you in advance.
Vostro 3910 GPU and PSU