Index/Foreign Key Question

List,
    I'm creating this multi company POS database.
My inventory table looks like (all items are unique):

id,category_id,invoice_id,x,y,z,gid,uid

I have a primary key on id, and then an foreign keys on category_id and
invoice_id.
GID is the group ID of the company, UID is the companies user, they are also
connected via foreign key to the respective tables.  My question is this: Do
I need to create more indexes on this table when inventory selects look like

select * from inventory where
 category_id = 1 and invoice_id is null and gid = 2

So where would the indexes need to be placed?  Or since I have the FK setup
are the indexes already in place?  I expect to soon have >500K items in the
inventory table and don't want it to slow down.  I'll have the same type of
issue with clients, invoices, purchase_orders and perhaps more

Ideas?

Thanks!

David Busby
Systems Engineer