No doubt about it, you really can not upgrade that system without BOTH a power supply and a new video card (GPU)......
Even if you have the 305watt supply, it certainly no Longer outputs full power due to it's age (capacitors just aren't what they used to be...). It's likely more like 250watts or less which won't allow the addition of any new GPU.
I would double check to assure you have a full PCI-E expansion slot on that motherboard. List detailed model number of the MB and we can help confirm what you have
Here's a list of power supply size requirement for GPU's: (no cards can be supported with less than 300watts..)http://www.realhardtechx.com/index_archivos/Page362.htm