I do agree that the HD 4870 scales well with AF and AA. And the 1 GB really helps. It's shown that 512 mb is holding the AMD card back in more demanding games at high resolutions and AA. (For example Stalker: Clear Sky)
Żet, you've made a good choice with the GTX 260 (216) which is also an excellent card. There are no bad cards in this videogeneration and I hope the GTX 260 gives you the performance you were looking for.