Well, if it genuinely is Rift, then there's not much you can do other than report it to the developer and/or Nvidia and hope they can patch it.
A couple of other things you can check, though:
1) make sure there aren't any background programs running that are taking up a lot of resources. In particular, security software like virus scanners can gum up the works but most of the better ones now have a "silent/gaming mode" which can greatly reduce the chance of it kicking up a fuss. I love Avast! for this in particular.
2) as it's an MMO, you'll be relying on network speed - not so much the final upload/download speed, as the latency. This can slow things down to make it *seem* like a framerate drop, so make sure you're not downloading anything else while you play. Also, surreptitiously remove the power supplies/batteries from everybody else's devices that could be using it too.
3) again, as it's an MMO, Rift requires more RAM than most games to run nicely. This may well be where a cheaper card will fall down. You can mitigate this by dropping down any settings for draw distance and caching things like decals or bullet holes (ok, so you won't be getting the latter in Rift, but you get the idea).