Found this on the Rift forums http://forums.riftgame.com/showthread.php?68673-performance-fix-work-around
you will find a download link to a dx9.dll, this dll was used by people like me when playing fallout new vegas, this game used the same gamebro engine and version, and i had the same poor performance for my ATI, all you do is place this DLL in your rift folder and it will be loaded, you should see an instant frame rate boost.
it works for me, i hope if work for you, use at your own risk.
edit to add better links NVIDIA: http://newvegasnexus.com/downloads/file.php?id=34778 ATI: http://newvegasnexus.com/downloads/file.php?id=34970
For Ati and nvidia> D3D9Gen_b3-34970 http://newvegasnexus.com/downloads/file.php?id=34970
use this, it will allow you to make a Dll with a selected card, i use a HD 5970, i asked it to make the dll for that card, i used this dll with the game and got even more performance out of it.
Just stick it where the rift.exe is and it should improve performance by a notable amount. I was able to up a few settings and still get better FPS than I was before.
The 3rd link requires a nexus account as its bigger than 2mb, but apparently it creates a custom DLL file best fit for your set up.
I would have said OC your CPU just by changing setting in your BIOS and adjust your FSB Frequency. I have a slow AMDx2 3800+ 2.0 (OC 2.3) and any extra omph really helps my framerate letting me run it in higher settings. For many AMD's you can increase your FSB Frequency by 20 without doing and damage and increasing temp by 5-10 degrees. If you are worried about overheat use PC Probe and it will warn you if it does or want to set a benchmark for normal temp b4 basic OC.
Current games: Rift (PC), Starcraft II (PC) XBL: DyinGLifE