How big for a system drive (PT rig)

[ProPower]

I currently run an HD2 accel PT system on a 2GHz G5 dual (power PC) machine with 3.5G of Ram...

I have a 250G main drive and a 160G Audio drive. The main drive is split into two partitions; one for the the OS and VI sample libraries and the other for itunes (I have my whole disc collection in apple lossless -- I A/B mixes against this all the time). Both the OS and the Itunes partitions are close to 80% full. Being an "old" computer guy I get nervous about the system partition getting larger and larger. I really want to stay with just a two drive system if possible (keeps my BU scheme of two mirror drives in place).

SO;
1) Any problem with going let's say 400G or 500G as my main drive with the same split in half, two partition scheme?
2) Do I really need to go to 3 partitions (OS, samples, itunes) and keep the OS one as small as possible (let's say 80G or so)?
2a) Would be much better to have a dedicated OS only drive, samples drive and Itunes drive (kind of a BU nightmare to me but....)
3) Any known issues with particular 400G-500G SATA II HDs in my G5, Dual 2GHz (Power PC). Just from past experience I was thinking of going for a Western Digital Caviar SE16 WD5000KS 500GB 7200 RPM 16MB Cache SATA 3.0Gb/s Hard Drive.

Thanks in advance for any comments or assistance,
-Lee