Hi Harry,
The Designjet 5500 printer was a "higher volume" graphics printer which is why the cartridges have a a capacity of 775ml.
Technology has moved on and the Designjet T770 printer is geared more to lower volume printing (130ml ink capacity). You will be able to achieve high quality prints but depending on your current output it could become quite expensive to run in comparison to the current replacements for your 5500.
the HP Designjet 5500 was superceded by the Z6100, Z6200 and the current models are the Z6600 and Z6800 both 60" printers.
(the Z6200 still comes in 42")
these are the models you need to investigate if you are looking for a direct replacement for the 5500.
regards
Doug Hing