New computer 2.8/3.0/3.2 which manufacturer ,Ram,Video Card

Thank you for all the previous comments, here are a couple more questions:
If I am buying a new computer either 2.8 or 3.0 or 3.2
What manufacturer should I buy for another 2gigs of ram and where to buy?
Which graphics card? NVIDIA GeForce 8800 GT 512MB or should I buy one from another vendor other than what Apple offers?
Does FCP make use of all these things?
Also I do a lot of converting from HD to other smaller formats through Compressor. Does an AJA $1800 card do a better job? Thank you for any extra help!

If I am buying a new computer either 2.8 or 3.0 or 3.2

Any of these would make a great base for editing. I always suggest you get the fastest on you can comfortably afford.

What manufacturer should I buy for another 2gigs of ram and where to buy?

As long as the memory comes with a lifetime warranty and is Apple approved, there are many places to buy memory.

Which graphics card? NVIDIA GeForce 8800 GT 512MB or should I buy one from another vendor other than what Apple offers?

This one is important. You can not buy just any video card and install it in your MacPro. It is best to pay a little extra for the best card Apple offers.

Does FCP make use of all these things?

Not just FCP but Motion and Color do in a BIG way.

Also I do a lot of converting from HD to other smaller formats through Compressor. Does an AJA $1800 card do a better job?

The AJA Kona 3 card can UP-convert from SD to HD but down converting can be done in Compressor easily.