The AGP bus uses 32 bits. So what's the deal on various manufacturers reporting 64-bit/ 128-bit/ 256-bit on their video cards?? Obviously, it's not referring to the bus width. What's that number referring to? Here's one example (see where it says Memory Interface 128-bit):

http://www.newegg.com/Product/Product.asp?Item=N82E16814150098

I'm not sure but I believe that is refering to the bus between the GPU and the RAM on the card itself.

MSU

That would be correct.

So all things being equal, a card w/ 128-bit memory interface can process 2x as much data per clock tick, compared to a card that's only 64-bit. Is that the right way to interpret that?

Thats how I understand it - the bus bandwidth is twice as wide.

Yep, but speed of the memory and the gpu, plus pipelines, texture handling, anti-alias.

Under Construction

Yes but all things being equal 256 bit is preferable over 128 bit.This was a big thing with the 9800 Pro when they came out with a 128 bit edition and didn't make it clear that it was half the bus width of the original.They ended up naming the lower bus card the LE version.They also offered the people that bought one of the slower cards free upgrade to the 256 bit and apoligized for the confusion.This mainly involved Club 3d and Sapphire cards.