I'm going to take the route that this isn't a collaboration/publishing site as this isn't mentioned and wss is in the tag list. Pretty clunky in comparison to using a publishing site...
First choose the web part page template you'd like to use from:
C:\Program Files\Common
Files\Microsoft Shared\web server
extensions\12\TEMPLATE\1033\STS\DOCTEMP\SMARTPGS
Then set up a stream to the template and use SPFileCollection.Add() to add it to your document library. For example:
string newFilename = "newpage.aspx";
string templateFilename = "spstd1.aspx";
string hive = SPUtility.GetGenericSetupPath("TEMPLATE\\1033\\STS\\DOCTEMP\\SMARTPGS\\");
FileStream stream = new FileStream(hive + templateFilename, FileMode.Open);
using (SPSite site = new SPSite("http://sharepoint"))
using (SPWeb web = site.OpenWeb())
{
SPFolder libraryFolder = web.GetFolder("Document Library");
SPFileCollection files = libraryFolder.Files;
SPFile newFile = files.Add(newFilename, stream);
}
Note: This solution assumes you have the US SharePoint version installed that uses the 1033 language code. Just change the path if different.