First off, go to their website and dload the user manual. Those usually list the limitations of each form factor. General guidelines are that the power supply is almost always low power, which means videocard choice is limited to cards that don't require a 6 pin pci-ex connector. Anything that is not a tower or mini tower usually only accepts a low profile card at most.
The wildcard is if the pre-built uses a proprietary design or standard design. If it's proprietary, then you won't be able to just buy any power supply. That computer seems to have a 250W power supply and the manual says the motherboard on the desktop is rated for 50W on the videocard slot. The standard is 75W, so that makes me think this computer has proprietary parts. Take a look at the design of the power supply and see if it matches standard ATX designs.
Your upgrade options may be very limited here.