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
8G ram
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
thought 20-25 was pretty bad for any game....... And yeah it is an old PC but I'd hope I could at least pull a steady 35-45 with decent custom settings and not lose too much video performance.
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.
RIFT is pretty demanding if you want any decent setting. The game looks like shit without AA, and even with 2x AA you will get at least a 20+ FPS drop.