If you plan to make it a PHP script, you’re going to have major issues.
Even if you use your own PHP.ini, with modified memory limit values, DH limits your user to something like 90megs.
So using a php script, it’s not going to be possible for users to upload anything larger than about 70-80megs I’d say. Then you also have to consider the fact that more than one user might be uploading something at the same time, which even further reduces the overall file size limit.
Anyways, there is no clear answer on this subject, other than to say “don’t do it” or “find an alternative”. You could always try to code your script in perl too. That would affectively work to do what you want, without eating up all of your memory.
Someone else also recently posted a link to this website, in regards to a flash based uploading program: http://www.element-it.com/MultiPowUpload.aspx
Good luck either way