best estimation is your performance is right between a 460 and 470 with those 2 9800's...
So youd be looking in the range of the amd 6950 or the nvidia 560ti give or take.
The 9800GX2 was generally right around the GTX 280/HD4890 area so a GTX 460 would be slightly faster. The cards you recommend are good choices for a nice upgrade.
I say 6950 or Ti 560, If you play any physx games you could get the 560 and keep one 9800GTX+ as a physX card. You could also do the same with the 6950, assuming you aren't using Vista, though it requires some extra steps.