If you can’t upload a file larger than say 50MB or 150MB for then it is likely you are running into the max file upload size that SharePoint sets.
Microsoft Office SharePoint Server 2007 (MOSS 2007) supports file upload sizes of 2GB. This is really a SQL Server limitation since files are written to the database. There are some related settings in web.config for the upload.aspx that you could change, but mine are already set to 2GB so that would not do any good.
For additional information on uploading file in general and this point, you may want to check out this post.
To check or change what the max size set in MOSS is, just open your Central Administration for the installation you want to check. Then go to Application Management tab | Web application general settings link and you will get the Web Application General Settings page.
In the example below you can see that I set mine to 1.5GB or really 1500MB.
Keep in mind this is set at the site collection level, not the site level.