It depends on what resolution and settings you like to play your games at, ie, what is your system and display capable of.
The HD48-- series really shines with their massive shader pipes at really high resolutions with all the eye candy cranked up, like AA and all that. The cards perform so similar at the lower end of the resolution spectrum, where the GTX260 pretty much is faster at low resolution and low detail on the system (but what's the point of these cards if you're using those settings?). They're really close at mid range resolution and settings. But at high end settings, the HD4870 will pull away. You'll see this especially in DX10 enabled titles, resolution maxed out (we're talking 1080p at least) and AA enabled at least at 4x if not higher. You'll see the difference in the performance at those settings on these two cards.
So if you have a nice kick ass display that is big and capable of 1920x1080 or better, and your system has a CPU able to drive one of these cards instead of bottleneck it, then the HD4870 is the one to check out. If you're gaming at lower resolutions like 1280x1024 or 1680x1050, and not using big eyecandy like 4xAA or higher, then it doesn't matter which card you get, as both will be overkill for them (exception: Crysis, but this game sucks and doesn't count anyways).
I'd agree that the 4870 is perhaps slightly faster at higher resolutions but if you were going to use 22" or above monitor maxed out you'd need the 1gb version which is still almost identical in price to the GTX 260 216.
It depends on your motherboard. if you can go SLI pick the 260. If you have a crossfire capable board pick the 4870. If you decide to go with the 4870 try to get the 1GB version. The extra memory makes a difference at 1920x1200 and above. You can't go wrong with either card but if you don't care about SLI or Crossfire and are still looking at the 512 version and the standard 260 get the cheaper one.
alright well this all sounds good, I thank you all for the feedback. Just so you guys know I plan on playing games on a 22 inch monitor with 1680 by 1050. I also have seen that both cards can outperform each other in different games. I am happy with getting the 260, especially since i got it cheaper then the 4870.