im also looking forward to the 9 series equivalent, that is whether they choose to use the same naming scheme ati had.
the main reason though i would even really be considering an upgrade like that from my current 7800gt 256, is because i may be getting a th2g de after not too much longer, and using it to then drive 3*19" lcds, so my current framerates at a given resolution will be reduced to a third of what they are now, so 60fps is the bare minimum needed at all times, otherwise you get definite choppiness at points. if i wasnt getting a th2g de though, my 7800gt would still suffice for quite awhile longer, easily another year or 2.
if i game at 1280x1024 for a single display native res, then ill need to change my res to 800x600 for 3 displays to get very similar fps... and there arent many current games that allow below 640x480, either (it doesnt support below 640x480 anyhow).
anyhow... thats a real reason i can see to upgrade a still capable gpu, beyond just wanting to stay on the cutting edge... upgrading, just to make it so your games are still even playable, so you dont need to reduce every setting down to the bare minimum, just to avoid stuttering.