The X1650XT is probally your best bet. Dont worry about the 512mb. If you want to save a bit of money up, the 7800GS would give you the best possible upgrade, as it is the best AGP card at the moment. Coming from an 9550, either choice you decide with, you will get quite an performance upgrade.
You might want to check that power supply, just to make sure it will suffice. Ultra does not have the best reputation.