Haha, everything is true.
Easiest way would be just to go to a computer store, buy a PSU, the right pin one, if it's the 20pin, don't' get the 24pin, and vice versa, test it out on the Gateway, and if it works, keep it, if it doesn't, return it, and you will get to know if it's your PSU, or your Mobo.
I have tons of computers lying around for when I do troubleshooting for friends and family, so keeping a couple spares of everything is definitely good.
Just for shit's and giggles, try and take out the new ram, and put in the old ones if you still have it, it probably won't make a difference, but it's worth a try.
You can get those external cases, if it's for a IDE hardrive, and it's pretty much just a IDE/USB adapter, and the case acts as a external case.
Like this:
IDE:
http://cgi.ebay.com/BLACK-3-5-USB-2-...lenotsupported
SATA:
http://cgi.ebay.com/2-5-SATA-HDD-Har...lenotsupported
Those are some good things to get if you do troubleshooting like I do for just small deals and whatnot.
For instance, I recovered tons of crap for a couple bucks with a external enclosure, and the Geek Squad at Best Buy said they might be able to do it, for an excess of $200, then my friends asked how much they would charge for a clean wipe, and then they said an excess of $250...
They went into the store, so the driving out wasn't a factor.
Complete rip-off..