correct you can not run crossfire on that motherboard but its not needed with the resolution you game at..
at the resolution you game at.. the 5750 is more then enough power.. and should allow you to turn on most if not all the eye candy in games.. also your PSU is more then enough power.. at most that system wont use more then 300w.. and in game you will probably be around 250w. but with it being a generic brand psu im guessing.. you will probably want to change it.. look into corsair, PC power and cooling, or seasonic psu's..
the amount of power required will really depend on what you put in the upgraded system.. but unless you plan to go with an i7 and a bunch of hard drives.. 500w should be more then enough with a single gpu..
main/gaming/distributed computing rig
phenom II x4 940 @ 3.5ghz 1.476v, Biostar TForce 780g m2+,2x1GB corsair XMS2 and 2x512mb kingston generic ram 3GB total, evga 8800GT 1.1v mod (756/1890/1015), Antec 300 w/ 5 tri-cool fans on high. 3.2TB HD space
HTPC/distributed computing rig
AMD athlon 64 x2 6400+, Asus m2n-sli deluxe, 2x1GB Crucial Ballistix Tracer 2.4v, Powercolor 8800GT accelero S1 (630/1836/1050), Xclios Windtunnel version 1 case