You're not quite sure unless you check out if standard motherboards fit. This is a well worn question and cases are pretty cheap so you may have to budget a case.
Check out https://www.reddit.com/r/PCMasterRace/wiki/builds for build ideas. Cases are in the list too.
The model you gave me lead me to http://www.asrock.com/MB/SiS/K7S41/index.asp which is a bog standard small board. Is the new motherboard about the same size? Should be fine.
We have an old desktop computer, we do not know what the case is as there is no manufacturers name on it, but when I used the dxdiag, it came up System manufacturer - American megatrends inc. system model No. K7S41. AMD Athlon (tm)XP 1.1GHz. Memory 448mb. direct version - DirectX 11.
We obviously have more up to date laptops, but I would like to (as a hobby) put another motherboard in, to upgrade the CPU, RAM and maybe an SSD.
My question is, what size motherboard would the tower hold?, I would like at least 4gb RAM. I think it is a rock?
Thanks very much for any help and advice.