It seems to me that if you don't have an AGP slot you'll need to replace the PCI monitor card with a PCI graphics card, and plug the monitor into that.
There MIGHT be problems. It's usually recommended that you don't put another card right next to the Graphics card, esp. a soundcard. I can't remember of this is because of posiible overheating or something else.
You don't appear to have too much of a choice.
I put Graphics on slot 1, modem or network (whichever is smaller) into 3 modem/network into 3 and sound into 4.
Don't do this yet.
Wait to see if others have better advice.