I don't think pet tanks really work in Rift (dungeons), I'm afraid.

For the most flexible team, I'd suggest 5 x Clerics - you run around as 5 DPS when questing, absolutely nuking your targets, and switch to tank/healer/3 x DPS for dungeons.

They are by far the best team I've run and the cleric tanking is very easy.

As far as your PC goes, I have an i7-920 @ 4GHz, 12GB RAM, 2GB ATI-6950, running Win7-64 and it's fine.
I run two 1920x1200 screens with a 1+4 configuration.
I have a small number of video feeds and pass-through for 'healing from main'.
I do need to run clients 2-4 on the low render option to get decent frame rates but it's not a big deal.

Good luck