OK, this is silly, how many ROMs/ISOs your computer can hold depends on your Hard Drive space, you say you've got about a 12Gb HD, with about half in use, so you've got about 6Gb space free (get an exact amount by right clicking on the C: in My Computer and selecting Properties).
So if we say for examples sake, that a GBA ROM is about 6MB at the most, that would mean you'll be able to store about 1000 GBA ROMs on your machine if you don't put anything else extra on it.
And whatever people say, the fact that your Hard Drive is full of ROMs/MP3s/ISOs/whatever else you decide to download, is not going to cause your computer to crash. What does cause the computer to crash? The fact that people don't do their general "housekeeping" on their machines. If you run a defrag every now and then, and I'm not talking once a year, I mean about once a week, then it should keep your computer in a reasonable condition that you can keep the ROMs/ISOs for as long as you want.
Also, a quick VirusScan once a week wouldn't hurt.
So, in summary, how many ROMs can your machine hold?
Plenty to keep you going for a few months. If you want more, spend about £45 on a 250Gb HD from Novatech, and you'll be able to hold more than you can get your hands on.
Also, as long as you get your ROMs from a reliable source, you won't have to worry about virii.
Any more questions? Feel free to Google it.
Regards.
.BUT THERE ARE POSSIBLITES OF YOU GETTING VIRUSES