GTX260 There is not enough performance difference to really tell the difference in a real world situation, and I suspect the extra memory on a GTS260 may start to play a more importaint part in gameplay than the better memory speed of a HD4870 in times to come.
And do n`t flame! I`m using a HD4870 and am well pleased with it!
I'd say go with the 4870. More shader power, like 40% more give or take, and dispite less memory because of its architecture has better memory management over the 260. Unless there is a great deal in price go with the 4870.
HDR lighting will play a prominent role in future games. Shader power is important. Better memory management, and a higher over ocee on GDDR5 allows for alot of the memory difference in size. To me, its a close call. I think the 4870 will get much better over time, because of driver updating, and currently a hair faster. I also believe the 4870 is more forward looking, as it has havok, DX10.1 and the ability to do things like HDR lighting in superior ways. The G260 is more easily oceed, except the Gram, and trades blows at this time in most games. Tough call, great pricing on the G260 finally makes it worth while at its current price point. Thing is, there will soon be a 55nm refresh of this card, making it eol, and wont perform as well as the newer model. Also, at a lil over this price point, we will soon have the 4850x2, which will be superior to the G280, even at 55nm. This of course is MHO