RNDU4000 vs RNDP400U? Features or just performance?

The CPU is the difference.

Note also that the Pro 4 (RNDP4000) has the same features as the Ultra/Ultra Plus plus more. It has the same CPU as the Ultra 4 Plus. The Pro 4 has a 5 year warranty versus 3 years on the Ultra/Ultra Plus.

So what sort of usage would warrant needing to go for the faster model. I've found the Ultra for $399, but the Plus is $649, so a lot of extra money that would pay for another drive...

If Im the only user, running in a network with:

Cisco 3924 broadband router with telstra cable ultimate and 1000mb ethernet ports with the following devices connected by LAN cable:
Tivo
Pioneer VSX921
PC
Laptop

I've run out of ports on the Cisco, so will grab another 5 port 1000mb ethernet switch to daisy chain.

Plan is to move all music, video, data to the NAS. Mainly will be downloading movies and storing it to the NAS and then playing via the Tivo to the connected Plasma TV and perhaps using bittorent to find that video on the internet.

I reckon that is all pretty light usage, and not smashing the box, so assume the Ultra will be fine, and Ultra Plus a bit of overkill? Correct?

Yes the Ultra should be fine.