Just finished playing around with my setup and this is what I found out:
With 4870
Disconnected the following one at a time: scanner, memory card reader, monitor USB hub, second monitor (rebooted every single time a device was disconnected)
Left connected - Main monitor, speaker bar
Results - Still get the MM clicking and buzzing in iTunes
With 2600XT
Left connected - Main monitor, speaker bar
Result - Get the MM clicking and buzzing in iTunes
Connected the following one at a time: scanner, memory card reader, monitor USB hub, second monitor (rebooted every single time a device was connected)
Result - Still get the MM clicking and buzzing in iTunes
After reading some of the suggestions/links about grounding what I did next was go to Home Depot and purchased a few terminators for my cable splitters, terminated all unused coaxial ports and also grounded the cable splitter down at the basement and repeated my test.
With 2600XT
First impression with just the main monitor and speaker bar connected was that the MM clicking was gone but I still got the buzzing sound in iTunes while browsing through folders. Proceeded to reconnect the other equipment one at a time and rebooting after each device and still had the iTunes buzzing but the MM clicking was definitely gone now.
With 4870
Disconnected everything again and just left the main monitor and speaker bar connected, MM clicking is gone but iTunes buzz still around. Proceeded to reconnect all the other equipment one by one (reboot after each connect) and still got the same result where the MM mouse clicking is gone but the iTunes buzz was still around.
It seems that grounding the cable splitters did at least resolve the MM clicking issue, what I kinda don't understand is my TV tuner is network based so it's not directly connected to the Mac Pro. Why grounding the splitters helped?
(I should mention that I am connected to my router via wireless)
On another note just as an observation my Mac Pro is connected to a CyberPower 1285 UPS, with the 4870 in the system I can get 9 minutes out of it at 75% load with the MP, 24" and 19" monitors connected to it. With the 2600XT and the same hardware I am able to get 14 minutes of backup time at 50% load, didn't know the 4870 was such a power hungry card.
Would hate to see what my backup time and load percentage would be if the 4870 was running at full tilt.