Stores that provide custom built PMs?

[wiseguy27]

[Note: Again, a newbie question - I'm a PC user who's never "used" a Mac.]

I love the Apple store (USA) interface where I can customize the machine and immediately see the changes in the price. I'm looking to buy a new Power Mac but I would like to tweak the configuration to see which options seem good to me (both usage as well as price).

Since the Apple stored charges sales tax in every state, which is an additional burden for me, I'm thinking of buying a PM from another authorized seller.

I'd like to know:
1. which sellers in the US provide custom Power Macs and how they approach this (is it like the Apple online store or would I have to call them to provide the specs and check the prices or visit their store physically...)
2. do they provide almost all the options for custom configuration (like RAM, video card, DVD drive, etc.)?

I looked at a couple of online stores but could only find "pre-built" PM systems.

Additionally, any recommendations on getting cheaper prices and deals w.r.t specific stores would also be appreciated.

Thanks for your time!