it is a BIG problem that you dont have pci-e or agp. that makes it hard to get any performance in games.
i whould realy get a new PC if i where you. you dont have to spend alot of meney to get sommething that is 3x better then your pc. you could be luck to find a 3x better pc at the junkyard.
if you realy WANT a new supported graphic card. then maybe this one will do the job. http://www.itbankeurope.com/CLUB+3D+...+PCI+512MB+DVI
but for twice the money you can get a better PC at ebay.
and i dont think this card force you to buy a new PSU. how much Watt does it has. this graphic card dont use much watt because that it does not have its own power input.
it is getting the power from the pci slot only. cards today have 2x 6pin connectors and pci-e itself gives more power then pci itself.