lol I just had one of those "DUH" moments....most Dell motherboards have monitor support on board...if this is your case then that would be your third monitor connection.
Do you have a VGA or DVI or both type connections on your motherboard?
If so then connect your lowest resolution monitor to that and your higher end moniors to your Radeon 5670...each monitor can be setup to its resolution specs via display properties.
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