I was once in your situatuion, having only pci and agp to work with. All I can say is Im glad I saved the money and didnt buy a piece of crap agp or pci card. Honestly Those Things would be no better than that that computer already has. AGP and PCI 1.0 were probably never meant for any type of gaming. They suck.
Also I considered buying a new motherboard for my computer which was very much like yours. the problem is that those thin computers do not take any new atx or micro atx motherboards. You cannot upgrade the motherboard.
At this point I would just continue saving up your money for that 700 dollar computer. Anything you try to buy for that optiplex gx270 either will not fit,will not work, or will not improve performance at all. IT would simply be a waste of money.
Sorry for the bad news but I dont like seeing people waste money. Try to stay away from small form factor computers like the optiplex in the future.