Would it be worth it to get something like described below for like $1900 and try to upgrade the RAM and add some 500GB HDs? I'm not sure how expensive those 2 things would be to add on--if that is going to take me up to what it would be to buy one of the new MacPros, for like 3K. (this one has apple care too):
2x 64-bit Intel Xeon Dual Core 2.66GHz (2 Processors, 2.66GHz Intel Xeon Dual Core. 4 Cores Total)

1GB 667MHz DDR2 ECC Ram Error Correction Code Ram (2x 512MB)

250 GB Hard Drive Space (Bay 1: 250GB Bay 2: Empty Bay 3: Empty Bay 4: Empty)

NVIDIA GeForce 7300 GT Video Card (256MB, Can power up to 2 montiors, including 2x 30" Monitors)

I think 1900 dollars is a bit on the expensive side... considering the 300 dollar difference for a NEW machine, that's a bit faster (I'm talking about the single quadcore option), has more RAM, a bigger hard drive, and a newer video card, and better front bus speed...

Don't buy RAM from Apple, they overcharge.

the new macpro with a single quad core and 2gb ram is still like $2800 (with applecare)...

is it a big deal to put some 500GB HDs inside the MacPro?

The new Mac Pro with a single core is 2300 dollars, I looked at the Apple website... Of course with Apple care, it's more, but you can't compare that, that's an unfair comparison, then you should account for the price of Apple care in the second hand Mac Pro too (where you would need it more too).

You have this site Mac2sell.net
They quote the price for a 2.66 ghz Mac Pro, and the specifications you said, at 1500-1600 dollars... Don't know if they are cheap, but I think 1900 dollars is a bit steep too...
I would definately go for 1700 dollars at the most...

Your choice of course.

Thanks for the feedback, I'll check out that site! One other quick question:

Best place to buy RAM for Macs?

Buy a Mac Pro with the standard RAM and then get the extra RAM memory from Kingston. The new 800 MHz RAM is already available. Here is the link.

Mac Pro 2008 Kingston RAM (http://www.ec.kingston.com/ecom/configurator_new/modelsinfo.asp?SysID=43378&mfr=Apple&model=Mac+Pro+Workstation+%282008%29&root=&LinkBack=&Sys=43378-Apple-Mac+Pro+Workstation+%282008%29&distributor=0&submit1=Search)