from anandtech's review about whats new in the 7xxx series (probably alot of other small things I just missed):
Directx11.1 (instead of just 11) (windows 8)
-Target Independent Rasterization
-formalization of Stereo 3D support into the D3D API
-some new buffer commands
WDDM 1.2 +
DXGI 1.2
-
Partially Resident Textures:
"The concept behind PRT/Megatexture is that rather than being treated as singular entities, due to their size textures should be broken down into smaller tiles, and then the tiles can be used as necessary. If a complete texture isn’t needed, then rather than loading the entire texture only the relevant tiles can be loaded while the irrelevant tiles can be skipped or loaded at a low quality. Ultimately this technology is designed to improve texture streaming by streaming tiles instead of whole textures, reducing the amount of unnecessary texture data that is streamed."
-
Eyefinity Technology 2.0
-
Discrete Digital Multi-Point Audio (DDMA)
-
Video Codec Engine (VCE) (kinda like Intel's Quick Sync, but able to put out much higher quality)
-
Steady Video 2.0
-
ZeroCore Power (insane low idle mode power usage compaired to other cards)
-Improved image quality with Anisotropic Filtering.
card:
PCI express 3.0
DP 1.2 and HDMI 1.4a (/w Fast HDMI technology) (support for 4K x 2K displays, ect)
"As next-generation TVs add support for 1080P frame packed S3D at 120fps (60Hz/eye)
Southern Islands products will be the first AMD products able to drive them over HDMI through the use of Fast HDMI."
I feel confident some of these things wont be found on a 480.
On the other hand, on a 7850, you wont find physX or Cuda.
On the short term, most of these things probably dont matter that much...
but if you plan on keeping the card a year or two, they probably will.