Need help! Used Space in dB and Tran files

I have a database about 39GB in size and is running out of allocated space. In the Enterprise Manager the properties for this dB, show that there is 0.00MB space available. I need to increase the size of the dB.

What I need to find out is, which file group is full so I can expand the appropriate file group instead of expanding all the filegroups. Also, is there a way to find the space usage of each file in the file group and also the Transaction log?

db Size 39GB
14 File Groups
17 database files (mdf & ndf)
2 Transaction Log files
180 Tables

I am aware of the sp_spaceused, but the information it furnishes is not enough for me to make a decision.

Need help urgently!!!!check this query out and see if this will be of any help:

select [Allocated_Size_MB]=size*8/1024, [Max_Size_MB]=maxsize*8/1024, name from sysfiles|||Thanks for your reply. The results from the query give me the allocated size.
How can I find the "used" space (or free/available space) in this allocated space?