I'm running it on an e7200 @ 3.16ghz, with 4gb ram and a GTX560 (non-Ti). It runs well enough -- most of the time it stays comfortably in the 30-40 range when outdoors, and only when it's streaming in new areas or displaying an absolute arseload of characters at once does it dip down to 25 or so.
Sure, you won't get 60fps unless you like pointing the camera at the floor, but I've just hit chapter three (and I took the apparently more computer-intensive path in chapter two) and it's never become unplayable. If you're upgrading anyway I say go for it.
I actually threw the 560 into this thing specifically to play this game, with the idea that if it didn't run well enough I could match it up with a new CPU/motherboard/etc afterwards. So far I'm happy, and the 560 is a pretty strong GPU so I'm going to wait to sling in a quad core until the end of the year. My settings are pretty much ultra with no uber or motion blur, and the dangling objects limit turned on and decals on medium because they're apparently CPU-dependent settings.