What do you mean by power? Performance will be close, the 5750 is a hair faster if I remember correctly. In terms of actual power usage its not even close. The 5750 has the power consumption of the 8800GT, much less then the GTS250. Add in Eyefinity support and DX11 and the 5750 is the better buy.
The 5750 has the power consumption of the 8800GT
You are thinking of the HD5770 I believe. The HD5750 actually uses significantly less than that.
Are you sure? I thought the 5750 is 8800GT in power but GTS250 in performance. The 5770 is between those two in power, but GTX260 in performance. I might have the numbers wrong. I do know the 5750 is in the 80sW range, so its not very power hungry at all. I wish they'd release a clocked down version that didn't need a 6pin connector. But that would remove just about all market for a 5670.