Fish, you might have a problem integrating the mobo into a propriatary case. I'm not sure how gateway does it, but compaq, HP amongst others have different connections for power button, reset switch, HDD LED, ect...which wont work with standard ATX mobos.
So, you might be looking at having to get another case. Unless you want to spend the time with a magnifying glass, soldering iron, a trip to radio shack for parts, just to adapt the propriatary connectors to use on the new Mobo so you can just turn it on.
You can get a new case with power suplly for around 30-60 dollars.
I would reccomend a 400-450 watt power supply for any "growth" that you would like to do down the road.
Also, going with you own case, ect...will make upgrading things easier than trying to stay within the bounds of propriatery computer.
You'll have more options down the road.
I'm redoing a friends pc right now, well in the planning stages. He's going to purchase a new case, Mobo processor and ram, and probably be looking at a new video card..middle of road.
Figuring we can knock it out for around $500-600. Looking at a p4, 2.8ghz, 1gb ram, decent 128mb vid card and new case. Ram is the one thing bringing the price up so much.
Anyway, basically the only thing we're salvaging from his old pc are the Harddrives, and CDR ,burner and dvd