i dont think the 3870's are 512. i think what your thinking of is the 3870x2 which has 2 gpus, so 2 x 256. many sites report the spec as 512

improvements with new buses are hard to say at the beggining. because the tech is new and etc etc... but right now if you compare a pci video card to a pci-e 2.0 you will see a huge difference from the extra lanes.

There are special 512-bit versions of the 3870.  I think our good friends at Diamond even make one.  I read a review somewhere and there was virtually no difference.  There's also no difference between the 256-bit and 512-bit 2900 Pros.  512-bit was a big advertising gimmick for the 2900 line, but it didn't do anything in reality. 

Crisis Causer:

There are special 512-bit versions of the 3870.  I think our good friends at Diamond even make one.  I read a review somewhere and there was virtually no difference.  There's also no difference between the 256-bit and 512-bit 2900 Pros.  512-bit was a big advertising gimmick for the 2900 line, but it didn't do anything in reality. 

Well, we got some more release date information:

http://www.tomshardware.com/news/ati-radeon-computex,5587.html 

"Two updated dates for your calendar are: June 17: Nvidia launches GeForce GTX 200 series. June 23: AMD launches Radeon 4800 series."

I didn't think Nvidia would beat AMD to the punch, but it appears they will.  Bummer.  If the 4800 is slower, which is predicted, it will hurt to come out later (even if by only a week).  But AMD may yet have the price:performance ratio advantage, we'll see.