Any time you try to install a video card into an OEM computer, you need to change the power supply. OEM power supplies are spec'ed to the absolute minimum needed to do the job in the system they're selling. So adding power hungry video cards can and will push them over the edge.
Determine what size power supply you have, either ATX or Micro ATX (M-ATX). HOPEFULLY you have a full size ATX power supply and replacing it will be simple. There are fewer choices for M-ATX power supplies, so...if you find yourself stuck with one of those, sometimes you're better off taking everything out of your current case and putting it into a new ATX compatible case instead of suffering with a M-ATX power supply.
I will say this. Your power supply is the foundation of your "computer house". Bad foundation = bad house. There is no better place to spend money freely than on a quality power supply. Personally, I don't think I've bought a power supply for one of my own systems that cost me less than $100. A good power supply should last you 5-10 years. Junk ones less than half.