Thanks for the input guys. I ran the diagnostics and it came up with no errors, so I opened the computer up and tested each individually.
My ram slots are labelled from DIMM 1 to DIMM 8, and alternate from black to white (odd numbered ones are black, even numbered ones are white). I tried putting everything in the white slots, but the computer didn't boot so I returned them all to the black slots.
I then took out all four RAM sticks and tried them individually on the black DIMM 1 slot. Three out of four times, the computer booted successfully and reported 1GB of RAM. In the last one, the computer refused to boot. So I guess I have a defective RAM stick?
What I did next was put in all the RAM sticks that were working. Even though I had 3GB of working RAM, the computer still reported 2 GB of RAM. I tried the following configurations: (1,3,5) and (1,3,7) but it still reported 2GB of RAM. So is one of the slots broken too? Will trying (1,5,7) work?
Again, I think this issue happened when I ran into an error with Seatools, and my BIOS settings got messed up. I changed the values again based on the BIOS settings on an identical computer, but I think there's still problems since the two computers boot differently (one shows a splash screen while another doesn't, even though their BIOS settings are identical as far as I know). SIW also has a warning symbol next to "Memory Timings", so maybe there's an issue with that too? The computer I have identical to the one with issues does not have this warning symbol.
Thanks for your help.