Have you tried testing the RAM sticks individually by booting with only one at a time? That's the first thing I would do.
Also, it wouldn't hurt to replace the CMOS battery on a machine that old. If it died and the system "forgot" its devices, or the BIOS file became corrupted, that could be causing all kinds of problems like that.
At any rate, I'd try the easy/free troubleshooting methods like that first.