your situation is kind of similar to mine, i need a new video card, but i currently have an agp 6600gt in a dell. I need want to play some of the latest multiplayer games as whenever i play cs i get owned by some random person called penis on fire, maybe its cause cs has been around for a while, so by starting a new game, i get a new chance.
i debated quite a bit on what to do, whether buy a temp card or not, and the conclusion was to just save for a new pci-e rig, for you it shouldnt be much. if you are willing to spend 250 now, then lets say 100 for a new mobo, and you can save for lets until you have a lil more to spend on the video card, atleast for a 7900gt. you probably will be getting twice the performance than any current agp card. you really only need 150 more dollars.