If you check your motherboard and its manual. You need to find out what type of memory it takes, how many slots it has for memory and how much memory it can take. For example my motherboard ONLY takes DDR 400, it only has 2 slots and only allows up to 1.5gig. So i have 2 sticks of 512mb DDR 400 but i could have a 512 and 1 gig if I wanted but no more.
I don't think the problem was 'it can't run 1 stick of memory' it was probably a density issue or the DIMM was too big. A lot of semi old motherboards can't use 1GB sticks of RAM, it will all be in the manual or on the manufacturers website.