On Pages 61 and 62 is a list of compatible power supply units (PSU) and instructions for replacing it. As you will see, there is a 300W PSU that is compatible. From what I can tell from the picture of the system board it appears that the power connector is a standard 24-pin connector. If this is the correct, you may be able to buy any standard ATX PSU that will fit into the space. You can determine this by measuring the existing PSU and comparing the measurements to the one that you choose. The only problem may be that HP has used other proprietary connectors. I have no way of knowing if this is the case. You can use HP PartSurfer to see if HP lists a 300W PSU for your computer: http://partsurfer.hp.com/Search.aspx?searchText=C1N64-67901
You will not be able to find any retail video card that will work with the current 180W PSU. Many of them require at least 300W to operate properly and this is actually the very minimum. Most of them require at least 400W.
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