I don't know if things have changed since I played, But IMO a game like Rift whose main selling point was that it offered These fantastic Dynamic Graphics detailed models and effects - So running it on low is just pointless..
I have a AMD Black Edition - AMD Phenom II X4 3.4 GHz Processor running at 3.8
And Sapphire Ati Radeon HD 5870 - 1GB
I tried on Ultra for a while it was ok, I had shadows turned off though...It would run at around 55 than drop to about 30 and sometime lowers
when things got very busy,
But I think there was a driver issue with ATI and Rift ( as Nvidia is one of the rift Cash cows ) so Nvidia cards seem to cope better, this might have changed
On a 3 year old system i drop down to less then 10 at times and i can still play with a shit load of people near me. FPS is often misused benchmark, if the game doesn't stutter and still looks good FPS doesn't really matter.