What are the speaker placements for 7.1 HT setup?

Dear list,

First and foremost, a great thanks in advance for all
helps in all forms.

I'm in the process of setting up a separate multi channel
audio system for HT. I have acquired the following
equipment for the purpose that fits my budget and taste.

1) Sony DVP-NS999ES DVD player.
2) Mcintosh MX134 pre-amp/processor
3) 3 MC1201 monoblocks.
4) 2 MC602 stereo amp.
5) Monster THX 1000 cables.

I haven't committed any plunder I hope?

I'm in the process of speaker selection for above setup.
Now, the MX134 is capable of 7.1 surround processing, how
many center speakers do I need? What are the correct speaker
placements for 7.1? Do I have a rear center? Is 7.1 better
than 6.1, why?