I think the rating of PSU (in term of watts) might not be a good way to determine if you system can handle some components upgrade. In the case of changing MB, it is ok to use your current 300W PSU since it is working fine with your current MB. However, the R9700Pro may load your PSU a bit more than the original Radeon. If your PSU is a good PSU, then it should be fine. Check to see first if your psu is a good one. If it is, say, Antec, Enermax, or Vantec, then you should be fine. If you can spend some money on a new good and quiet PSU, then go for it. The Antec True430 is nice and quiet and cost around $85 shiped (True380 cost a bit less and you might get it for $80 shiped).
Nvidia or ATI, it's all depend on what you like and system optimization