I'd like to know if I need to be concerned about any hardware parts that I should include in a brand new Mac Pro purchase, instead of trying to hunt it down on 3rd party vendors (or whitelist vendors / hardware suppliers).

The main components I'm interested for "upgradeability" are:

• Processors (If starting with Two 2.4GHz Quad-Core "Westmere");
• RAM (If starting with the least possible, which seems to be 6 x 1GB);
• Video Cards (If starting with one ATI Radeon HD 5770, can a 2nd one be purchased elsewhere?)
• Hard-Drives (Since these are mounted in specialized trays [if I'm not mistaking], are they also sold elsewhere? And can they be bought as SSDs?)
• Power Supply (Do I need to be concerned about this at all, or does it auto-adjust depending on the new component upgrades?)

I just want to be sure by choosing a Mac Pro with lower component specifications that I can in fact purchase upgrade parts cheaper elsewhere.

Thanks!