with only 45 it sounds like it's probably ok. In actuality you only need 3 or 4 to run XP, ~6 with networking, but i won't get into that.
with all the swaps you've tried it does sound like a mobo issue. You could use the same one, there's no issue there, when i built this computer i had a fried mobo and processor, but after getting them replaced i'm more than satisfied with the result, sometimes mass production makes a mistake, it sucks, but you just hope it's still under warranty
how many dims of RAM are you using, and how many slots are on your board? Also which type of ddr are you using? PC333, PC133, etc?