If you can swing a 5850, that would probably be better, gaming-wise.
However, I had a GeForce 7600 GT that had a 128-bit interface and didn't have many/any stutters while running it, but my newer 9600 GSO with 128-bit memory will stutter if AH details are turned up too high...
It's more the card than the memory bandwidth. A total-package deal, rather than "oh, 128-bit is slow," it depends on what card.
Just speaking from general knowledge of the numbering systems, If you can't get a 5870 or a 5850, you might get better gaming performance from a 4870 instead... The numbering on ATI cards sometimes confuses me, but the x600 lines are definitely budget, the x800 lines are their gaming cards, and I'd assume their x700 series is the cut-rate cards (hence the weaker ram). You may dodge the bullet and get the "older" gaming card, knowing it'll be better for gaming, rather than a "newer" card that you aren't sure about.
The clincher is if you want DX11 support or not -- this is the one with support for up to 6 monitors (let's hope we're not running off vista or Win7 by then). Note, though, DX11 isn't out yet as far as I know, and is a ways off.
EDIT: I did a little checking, a 5850 will out-pace this card in most specs, despite a slightly slower clock on the GPU and the memory. 5770's about par (specs wise, not sure in-game) with a 4870, but a 4890 (if you can find one, the big brother to the 4870) will outpace it again.