Your Video Card upgrade depends on if it will fit the internal dimensions of your case, if you have a power supply that supports the power demands of the video card and has the proper 12V molex connections.
What games will you be running? so we get the whole picture of what you need for processing CPU/GPU power and RAM. The Core i3 might be plenty or a bottleneck depending on what your planning on running. A Core i5 would be a better start for a gaming rig with a powerful GPU, however the i3 might be enough depending on what game you plan to run.
If this is a off the shelf computer such as a DELL, HP, etc please supply the Make/Model# ... if this is a custom built computer please provide all specs including internal case dimensions or case make/model used to look up dimensions as well as power supply make/model which will show if its a good quality trusted power supply or one that should be swapped out as well as if its heavy enough to carry the load.
You mention you have a replacement video card in there now... what are you running as your replacement temporary solution?
You also want to get by as cheap as possible... what is your budget?
