We hear that a lot about 'little brother' GPUs, and in the end, it's a mis-leading statement. Generally speaking, consumers using high end graphics cards, are also running very demanding 3d applications. And within these applications, they are applying features such as vsync, supersampling transparency anti-aliasing, increased HDR etc at high resolutions. These things will punish mid or low range GPUs. However, when these features are absent, especially at lower resolutions, cards such as the 280 and 4870, will perform marginally better than a 260/4850 and older cards like the 8800 or 3xxx series; and as well as the mid range products in those line ups.
Try to ultra max out Crysis, AoC, WiC, Mass Effect etc. at 1920+ resolution, and you will definatley see a difference between a 260 and a 280.
A card like the 260 exists because on the surface it is - "GTX260 can reach near GTX280 performance once OC'd." But it's cheaper because deeper down, that claim is NOT true.
The same went for the 8800 640 GTS and 512 GTS vs the 8800GTX.
It's a good price/performance card, but again, only on the surface.
A better question is what's in the computing future?
Nehalem
Native DDr3 boards
Ray Tracing capable GPUs
3d applications and games making use of physics dies on GPUs
However ray tracing and physics is still undetermined in terms of 'when.'
Nehalem on the other hand is very real and is coming soon; meaning until applications start making use of a GPU's physics die, the architectural change that Nehalem brings about, might make a signficant contribution to helping GPUs 'stretch their legs' as it were.
Finally meaning, the 4800 and GT200 series will be worth getting into, rather than waiting six months for the next product line-up from either GPU camp.