Probably........but I am not sure I would try it. Enermax PSUs are generally quite good however the only review I can find for this one is from 2003. If its that old I might consider replacing it with something newer.http://www.cluboverclocker.com/reviews/power/enermax/eg465p-ve/p2.htm
As you can see the test system includes a screaming fast Ti 4200 8x agp card
Either of these are a great replacement and also very high quality. Either should run a single GTS 250 with no problems. The 550 will give you some extra headroom though.
Corsair 450VX http://www.newegg.com/Product/Product.aspx?Item=N82E16817139003
Corsair 550 VX http://www.newegg.com/Product/Product.aspx?Item=N82E16817139004