Creating more storage space

Hi,
currently on  one our our file servers where all the users folders are located the drive is nearly running out of space so wanted to know best way to solve this issue.  The size currently taken up by the users folders is approximately 275gig and there is about 25gig left.  I dont wana move them to another server if possible.  Any ideas?  Sans?  Nas? Thanks.  This is on a windows 2003 standard server.