"Swap-file" thingy - Helps to "assign" it to 1 hard drive and use the

...other for storage?

Hi, I read in a few other posts that the Swap File is the thing the computer uses on your Hard Drive
when the physical memory gets full, and that if you have 2 different Hard drives on your
computer (not a RAID array - 2 different drives...)
that you can
tell your computer to use 1 hard Drive for the Swap File and you can use the other
to run your programs off of, save things to, and use pretty much for everything else...

Is the performance gain here huge or small? Is it worth getting a 10,000rpm Raptor 74 Gig. Hard Drive just to put the Swap File on and use for NOTHING ELSE?

Is it worth having 3 Different Hard Drives and using 1 for the swap file, 1 for your programs, and 1 for Data storage?
Does the cost in
$s to buy all these drives get you real-world improved performance???
(or is it a waste of time/money?)

Thanks, (as you can tell from my post I really know nothing about the Swap File, how it works/what exactly it does, etc.)
I am building a new computer and would like to know what Hard Drives to get on it...I would like the "money-no object" advice and then
I'll just see if I can afford to go out and actually buy it!