Okay, I had all but given up on getting my first DIY project to work without replacing the Mobo. I comunicated with MSI twice and they said to first, try a bigger PSU, which I did and got myself a bigger case to boot, and two, try an AGP video card instead of my PCI. Neither of these worked.
Then as a lark, I put the second memory module in. I had been only running one for testing purposes. It started and loaded like it was supposed to have a month ago.
What I am at a loss to explain is how one module won't work and two will. Maybe it has something to do with needing two modules for the DDR to function?
Anyway I am now the proud owner of this:
AMD AthlonXP 1800+ w/ Thermaltake Slim Volcano 8
MSI 745 Ultra Mobo
384 MB of PC2700
12 GB total HDD
32x10x12x CD-R Pacific Digital
Geforce 4 MX 420 PCI w/64MB of video RAM
ATI TV Wonder VE
RealTek 10/100 NIC(comes with the cable modem)
It's not the latest or the greatest, but it's a damn sight better than what I had, so

BTW, thanks to everyone who answered my stupid and sometimes inane questions over the last few weeks, you guys are teh :thepimp: :thepimp: