I have been considering getting a Mini PCI-Express Graphics Card Adapter so I can get a desktop GPU, as well as maxing out the RAM with 8GB and replacing the 500GB HDD with a 3TB HDD.
Yeah, but a crummy processor and motherboard are still, well, crummy (no offense). I wouldn't doubt that slapping a desktop GPU on the thing and adding more RAM would certainly help, but that still leaves literally the heart of the issue remaining.
I'd honestly just take the money you'd use getting that laptop upgraded and build a cheap gaming rig. Depending on the kind of bargain hunting you can do on newegg or similar sites, you can build a gaming PC that at least packs as much of a punch as a Xbone/PS4 for about $400-$500 USD.