The problem with purchasing off-the-shelf systems for later upgrades is that OEMs often use crippled motherboards to save money. With some boards, you cannot upgrade the processor. Some boards do not even have a PCI-Ex16 slot. In your case, the system already states a maximum limit of 8GB of RAM. This is a motherboard limitation and not a RAM module limitation.http://us.gateway.com/gw/en/US/content/model/DT.GCFAA.002
Since this appears to be a small form factor PC, I doubt you'd be able to upgrade the power supply. You certainly would be limited to a low-profile PCI-E graphics card (assuming it actually has a PCI-E card slot). This is not a system you purchase for upgrades.