xfx,asus, evga there all good makes the only diff is sometimes the warrenty and bundle, sometimes build quality if say its not a refrence cooler,never had a pny i usaly stick to the 3 i mentiond but evga were my best but xfx arnt shoddy at all
they both should work fine mate,have you got a SLImobo? if you do then get a 275 if you dont get a 4890 IMO as its cheaper save the £30 put it towards a fermi
to find your power supply wattage you have to take the side of your computer and find something what looks like this and the wattage should be labeled on there