I doubt you'll find any detailed information on the differences between iTTL and D-TTL as Nikon has never divulged (to my knowledge) how either of them work (trade secret). All we really know is that they both use preflashes to calculate the exposure prior to taking a picture. I believe the intensity of the preflashes may be different (with i-TTL being brighter); but, that's about all we know. Apparently, D-TTL was not as reliable as iTTL which is why Nikon abandoned it and developed iTTL.
There is also regular TTL which is used by film SLR's. TTL doesn't use preflashes. It measures the light reflecting off of the film plane during the actual exposure and turns the flash off once it determines enough light was received. Apparently, light doesn't reflect off of digital sensors the same way it reflects off of film which is why Nikon (and others) have resorted to preflashes.
The important thing to know is that the camera body determines which is used. Nikon Film SLR's use TTL (with the exception of the F6 which, I believe, also supports iTTL), older Nikon DSLR's (e.g. D100) use D-TTL, and all of the current DSLR's (e.g. D50, D70/s, D80, D200, D2x, etc.) use iTTL. To my knowledge, there are no DSLR's that support both D-TTL and iTTL. I don't know about the D2h; although, I suspect it supports D-TTL.
Older Nikon flashes (e.g. SB-24/25/26/27/28) only support TTL; although, they can be used on your D2h in manual mode. There are a couple of flashes (e.g. SB-28DX, SB-80DX) which support both TTL and D-TTL. The SB-600/800 support TTL, D-TTL, and iTTL. If your D2h supports D-TTL then there really shouldn't be much difference between the SB-80DX and a SB-800. The fact that the SB-800 also supports iTTL is irrelevant since your D2h doesn't support it.
What I don't know is if you'll be able to use the wireless capability (I believe that is one of the enhancements they added with iTTL). The SB-800 also has a SU-4 mode; however, that might be tricky to use because it seems like the D-TTL preflashes would cause it to fire prematurely. However, you should be able to connect them together via sync cords. I believe both the SB-80DX and SB-800 have 3-pin TTL sync ports (via a SC-18/19/26/27 sync cord) and standard PC sync ports (via SC-11/15 sync cords). Although, I suspect you would only be able to use the remote flash in manual (or possibly Auto) mode.