PHP does have a limit on what size files can be uploaded through an online interface (like a form).
However, this can be overridden in the server settings if you have access to that (like on a dedicated server or friendly shared server host)
If business gets busy enough, you could try the branded version of yousendit.com. It works really well for all size files (up to like 2GB I think). You could also use the free version in the meantime.
Here's another one you could try:
MachForm - PHP HTML Form Builder - Mailer Form Creator