Having trouble importing posts from WordPress to WordPress Mu

I am exporting posts from a WordPress blog to be imported to WordPress Mu. Unfortunately, there are over 3700 posts, and while we've increased the upload file size limit to 256MB to accomodate the 50 MB file, the import only loads 400 or so posts before giving me this error:

Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 71 bytes) in /home/dealmom/domains/dealseekingmom.com/public_html/wp-includes/plugin.php on line 302

Is there another method I can use to move all of those posts, comments, categories and tags over?

Thank you!

Mandi Ehman, Project Administrator
Deal Seeking Mom