9800GTXs have slightly more than 160watt draw at load, so 3 would be around 500watts draw. Add in the CPU, HDDs, mobo, RAM, etc and it would seem that your 650w shouldn't have been able to run 3 9800GTXs. (or at least it's cutting it VERY close for comfort)
Two 4890s will use less power than 3 9800GTXs.
A 4890 is about equal to a GTX275, and both are about 15% better than a 4870/GTX260, which in turn is about 20% better than a 4850/9800GTX.
Two 4890s will probably perform slightly better than 3x9800GTS due to better scaling.
Thanks alot for the reply but should my 650w be able to take those 2 4890s? I mean it is a corsair =).