People going with the 4870's becasue on paper they're beating 260's but in real world it seems like a lot of peolpe are having issues with them. So they try the 260 out and with most don't look back after they do.
It's funny you mentioned that because that was the exact situation for me. I was torn between a 4870 and a GTX 260 (original). The HD4870 was the better performer and I came within inches of buying it but I ended up going with the GTX 260 mostly because of everything people were reporting on the net about the 4870.
I think most people report a max stable oc on the 260 of about 700 on the core. That's where I've left it at. Supposedly, the OC on the core yields more performance than oc'ed VRAM. At one point during oc'ing, I got a checkerboard-type pattern which scared the crap outta me so I'm not pushing it too hard anymore.