If you download and run cpu-z, it will tell you what your motherboard is. Primarily, as mactronix asked, does it have a pci-e slot, and agp slot, or none at all.
Look at the label on the psu. It should say what the +12v amps are. Does it have a 6 pin pci-e connector? At 300w, I would doubt it.
I would think that the best you can do is a vga card that is entirely powered by the pci-e slot like the 9500GT. There are some amd cards that would do it also.