It's hard to compare the memory clocks of those two particular cards, as they use different memory types and have different amounts of memory. But for reference the 4870 is probably a hair better than the GTX 260. I'd go with the 4870 if it were me picking and I'm sure most people on this forum would agree.
Ive heard and seen benchmarks of the 4870 even beating the 280gtx, and totally dominates the cheaper 260gtx. Diff go for the 4870 I just got one and its flippen awesome.
Heat issues are easily solved with the quick and easy fan fix you can do (google it to get a tutorial about it)