I tried shuffling around my memory cards and pulling them out one at a time, but no matter what configuration I tried, 4 memory cards caused BIOS to register 4GB but XP to register only 3.25GB, and 3 memory cards in any combination caused both to register 3GB.
I was going to try messing around with the graphics card next, but while I was putting the memory cards back, my 4 year old little brother decided to "help" and turned my system on while it was on its side.
I always assumed it was a bad idea to do that, but I had no idea how bad it was. Now, my system won't even start up. The CPU beeps loudly 3 times and the fan cranks up to full blast. It doesn't even display the Gateway logo or anything else on the monitor. I called Gateway, but all they could tell me was that my warranty expired in June.
I imagine I crashed the hard drive or something equally catastrophic, so thanks for your help, but I think it's a moot point now. Forunately, I still have an old clunker laying around, so I can still connect to the internet. I expect I'll have to take my computer in for repairs, but I'll keep checking this thread if you have any further advice. I would be forever grateful, but I'm expecting the worst.