you would be better off getting a new power supply
they arent that expensive
but it all depends on what card you want to run
which will give you a good idea about which psu to get
The motherboard in the HP a1646x has an available PCI Express x16 slot for a video card. The 300 watt power supply will be plenty for what you need.
Going with your 300W power supply, I would suggest getting a SAPPHIRE Radeon HD 2400PRO for only $29.99 ($19.99 after $10.00 Mail-In Rebate). It has both VGA and DVI outputs and will be well suited for working with photos.