you want a cheap rig that works fine at 720p?
Not a fan of the motherboard, but it is simply an example.
This is a very reasonable CPU with onboard graphics.
3600mhz is the peak speed for at least drop in and easy to manage speeds, just enable the XMP profile for memory.
This setup performs very well at 1080p gaming standards, and will quite easily smash anything at 720p
Look at the price as well, which includes a basic small NVME ssd drive for booting up fast.
One VERY LARGE downside to this rig.. There IS a potential for this not working out of the box, due to BIOS limitations.
the 3400G is a newer processor based on 2nd gen ryzen architecture. This means you will most likely need a friend that has a 1st or 2nd gen ryzen chip to drop in and use to upgrade the BIOS prior to using the 3400G.
One way to side step this is to buy a motherboard that has onboard ROM setup that allows a CPU-less BIOS update process. Researching a few motherboards will let you find this information.
This APU setup will run quite easily on even a 200wqatt power supply, so re-using your 400+watt supply will save you some money!
EDIT - going this route enables you to sell your current video card, as the onboard graphics will likely perform very similarily, and in some cases better, due to utilizing the 3600mhz ram.
That high ram speed aslo doubles as increased cpu performance due to ryzen cpu architecture standards