Read the manual
If your new Gateway came with any discs, the manual should be a file to print out or review on screen. If not, then visit the Gateway support website, find your model# and enter any info they require to check the actual S/N# of your PC to see what's what. Get the manual, download it and save you'll need it sooner or later.
Offhand, if you get the new video card, get a new PSU. That will support the demands of the new video card that will be very important over the long term usage. Having a PSU that very capable with extra wattage than your current PSU and being of a decent name brand helps considerably. Check the manual on the video card and/or visit its support website for details. I suggest you get a new PSU that at least 100W more than current Gateway one. usually, the video card will want X-watts available, so use that as a gauge. if it states, it needs 450W, get a 550W PSU. Cheap is cheap!