If your an Intel fan i would say go with the Sandy Brdige i5 or i7.If AMD go for the Phenom ii x4 970 or the Phenom ii x6 1090T(personally i'd go with the x6 1090T,but the 970 will work fine too.)Of course with either of the upgrade paths your going to need a new mobo.Now i know theirs been some problems with the new Intel mobo's but i think they are restocking them again.But i'm not sure because i'm an AMD fan.You'll have to check with someone else on that matter.
So with either upgrade paths your going to need New RAM,New Mobo.Unless your RAM is DDR3,but i'd guess it's DDR2.
Are you going to SLI the GTX570 in the future?It's always good to get a PSU that can handle that setup when you go shopping for one just in case.
Are you going to/into Overclocking?If so i would suggest one of the new Sandy Bridge i5's or i7's i've heard and seen them O.C. and they are really really good at O.C.ing.Just thought you should know if you are into O.C.ing.