dunno bout for bikes but the tom tom is excellent.Gem has a good one too garmin i think it is.Would imagine you can get a bracket to fit the bikes off ebay?
Don't forget people there is more to the zxr400 than this forum... check out www.zxrworld.co.uk also.
I bought myself a Garmin i3, the only problem is that it isnt waterproof. It sits under my screen and if I get caught out in *really* heavy rain, I stop and put it in my bag/under my pillion seat.
I bought a mount which comprises an M8 bolt with a ball on the end to mount the GPS on to. I then made a bracket to attach the mounting bolt under my screen.
it's not the most advanced GPS, but for the price you cant complain! you can get them for £130 with maps for all of the UK
re tomtom vs Garmin: A friend of mine has both, and says that the Garmin is *much* more accurate when it comes to predicting your ETA,
if you want a cheap waterproof GPS, my other half has the Garmin eTrex legend. It comes with a base map of the entire of Europe with is good enough to get you home
it depends what you want from a GPS really, you can pick up very basic ones for not much money, but they are not much more use than a compass!
if you want cheap, and arent bothered about it being waterproof - the i3 is a great little GPS