I just went through the exact same dilemma lol. I was changing my mind all the time about which was better. I decided to go with the GTX 260 in the end and I'm very happy with it, its a great card. I decided on it mainly because it has PhyX although there arnt a great deal of games that support it yet.
From looking at benchmarks, the 4870 seems to do better when AA is added but not in all cases. Without AA the GTX 260 did better. Having said that, I found benchmarks very confusing cos nearly every website would have different results to the last, so it was hard to find a definite answer to which was better.
I have found the GTX 260 overclocks very nicely which is a major plus if your into that. Although I have never compared it to a 4870 so I dont know which is better in that department, maybe someone else could help you out here. Stock clocks are 576/1242/1000, I overclocked to 700/1511/1200 which I was rather impressed with and Im sure if I put the time in I could push it a bit higher too.
The only issue I have come across with this card is when I load up games and it has the little intro and what not at the beginning I get a squealing noise from the card. It only ever happens in high fps situations though. It happens in Crysis and a tiny bit in Far Cry 2 but not at all in less demanding games. When your actually playing a game you dont get the noise. There's no noise at all when doing anything else on the computer. Only at game start ups.
I looked it up and this is why:
This squealing is caused by vibration of copper coils, and is not present on products designed by people that take attention at these things. Read: if your card has Digital Voltage Regulation Module (DVRM, as Iwill originally called it - Digital PWM is more popular these days) or all solid-state caps and shielded chokes, no sound should be produced. But, if your part has coils or non-shielded capacitors/chokes, you could be “enjoying” in squealing sounds of electronics.
Here's the full article:
http://theovalich.wordpress.com/200...w-and-how-to-fix-it-no-more-gtx280-squealing/
From what I've read its a bit of hit and miss though, some of the GTX 200 series cards will do it while others dont. So just research the manufacturer you want to buy from first and see if other people have had the problem. It is fixable but will void your warranty.
Lastly I'll just add, if you buy a GTX 260 get the 216SP 55nm version
I personally haven't had a single issue worth mentioning with CCC or any other ATI software. My experience with nVidia's drivers hasn't been as pleasant unfortunately. Maybe I'm just a lucky/unfortunate one, but that's my experience with it.