Computer RAM question

I am trying to revive the old “mothership”. I think it has memory/RAM problems.  It crashes when I get past the log in screen.

There are four of these installed now:

1GB 2Rx8 PC2-6400U-666-12

Questions: Do I have to put precisely the same modules in pairs?

Can I increase the RAM by getting 2 2GB modules and keeping 2 1GBs?

Actually, since it is 32-bit, I think it can only take 4GB total.