I have the enjoyable task of replacing a hard drive on an ancient PC. This is a frankenstein system that was put together with bits and pieces of others. Is there a limit as to the size hard drive I can put in this antique? Running fine now - just want more storage space. It currently has a 640MB and I'm trying to upgrade it to a 6.4 GB but the BIOS won't recognize it. Is it the size or something completely different?