Most of them don't work or you need a serious powerful system to run them (3d games).
If you weed out the non working ones you'll be fine with that harddrive.
Beggers can't be choosers but i don't get why people are pumping full 100gb romsets around the internet and 50gb simply doesn't work in Mame. It should be cut in 2 parts for those who use other emulators.