If you got an agp socket and the motherboard is capable of using it then soldering it on could work if other componants arn't missing.
I have never seen a dell without an agp slot before. Even saw one of the newer p4 with the black case the other day and it had an agp slot. Perhaps only the really cheap ones don't have them.
I don't think it's worth the effort of soldering on a slot. If the holes are filled in you would need to drill them all out, and I wouldn't personally use a cheap soldering iron with the joints so close together and on a motherboard.
Just buy a new mother board and don't buy another really cheap mass produced pc without fully checking the specs again.
If you are going to throw the mobo away it would be intresting to see what happens if you solder on a socket.
dbpro : p166mmx @ 233 : 256mb : sb 128pci : sis onboard