To the OP.
first off, if you want to play Rift the best you need to avoid both AMD cpus and ATI. It is just the way it is right now. There are a few things you can change in the rift.cfg file right now to help out, but the game is not configured correctly for either of these. Worse so if you have both an AMD cpu and gpu right now. This is right now a problem with only the Rift MMO game coming out next week.
Also, your e8400 is just fine, especially if you overclock it up. Most e8400's were able to do 4GHz+ range fairly easily with decent air cooling. I high suggest you do this as it will save you some cash in the long run.
As for vid cards, the 560 or 570 will work well enough. More so actually than you think. You'll be at the limits of your CPU with those cards which is fine.
With an e8400 cpu @ >=4GHz + gtx 560 or 570 + >=4GB of mem you should have no problem in any game today with the exceptions of metro 2033 and crysis at the highest settings.