Personally I'd get a Seagate since I've had problems in the past with two different WD's but never with any of my Seagates.
As to network back-up I've got three systems set up as follows:
Laptop:
60Gb boot drive
60Gb shared storage drive
60Gb external drive
Desktop:
20Gb Win98SE boot drive
60Gb XP boot drive
120GB shared storage drive
120GB shared storage drive
120GB shared external drive
Desktop:
250Gb boot drive
250Gb storage drive
500Gb network back-up drive shared as needed for back-up
500Gb network back-up drive shared as needed for back-up
I personally think that as long as you have the original files/drive and a back-up copy on another drive no further redundancy is needed although a virus infection could potentially wipe out everything. That's why I only share my back-up drives during back-up and otherwise don't share any drives on that machine. Still, an infection to that particular machine could also spread to the network wiping me out.
I've never really had a virus so I don't worry too much about it and I don't have the time/inclination to back up to a complete spindle of CD's but if you really want to be safe just dissconnect the network back-up drive(s) when not in use. That gives you redundancy in two isolated locations.
BTW, I don't bother mirroring drives. I just back-up my settings and data. If I have a failure I'll reinstall the OS and programs.