I would say an 8800 would be your best best. You should fill your ram up (max of 4GB) and go with the best CPU you can for that socket...
No, to hell with that.
You need to rebuild. Sorry.
Just get what you can for what you have now or give it to your kid sister or donate it.
Invest a little here, a little there, and start fresh.
There is an excellent article here on Tom's Hardware for building a budget rig.http://www.tomshardware.com/reviews/core-i5-overclock-performance-gaming,3097.htmlEdit: I was wrong, the 8xxx series doesn't support AGP either.
You REALLY need to just rebuild.