There is no guarantee that ATI will put out some driver that really gives it some kind of advantage.
I personally don't bank on stuff I have no guarantee of. For all we know the drivers could get worse or just continue to suck.
Looking at the here and here, the 4870 and GTX 260 perform very much the same. Maybe the 4870 is 2 or 3 fps faster in some games, yet the 260 wins in other games.
On top of that (and not sure why this doesn't get mentioned), the GTX 260 is definitely the better overclocker. I know one person with a 30% overclock. I know 2 people who can't even reach a 10% OC with the 4870. When you add OC'ing in as a factor, which you damn well should since everybody and their mother does it (and companies like eVGA are even throwing in their own OC software), then the 260 is the better buy IMO, you really can't make a bad decision here.
The 260 also has the better cooler, and the better drivers right now. You here that the 4870 *will* have better drivers coming. Even if that is true, we need to consider that Nv will be putting out better drivers as well. And you never know when that Cuda will come in handy.
Also, with the software you mentioned you are running, you might want the 400mb more memory the 260 offers. Granted it isn't DDR5, but the 260's DDR3 is still plenty fast and its memory bandwidth is not all that far behind that of the 4870's.
If 512mb of memory is enough for you in all situations, then clearly DDR5 is the better choice. But let's remember that we already have games released that recommend 512mb.
Cut to 5 months from now and games/software will only be more vid ram hungry. There comes a point where if your rig is wanting a tad more vid ram, it doesn't matter how fast ddr5 is when you don't have enough of it. More ram can cure many things that faster ram can't.
I'm not telling you what to get, but rather helping you look at the entire big picture here. What is best for you might not be the best choice for somebody else, especially if you are considering SLI'ing one of these cards in the future.
The 260 definitely isn't a clear loser here. Nor is the 4870. If you run at lower resolutions, are fine with 512mb ram, and don't overclock, then perhaps the 4870 is your better choice. If you do run at higher rez's, think you'll want or need more than 512mb ram, and do want to overclock, then I say the 260 is the way to go.
Think about these things and you really can't make a wrong choice.