Normally I'd recommend you try the 5.0-dev builds first as it fixes bugs and makes the CPU thread faster, but the option for half resolutions (1.5x and 2.5x) has been removed because of the problems they cause, and the GPU thread workload has increased which may cause you to be stuck at 1x resolution.
As for the external express card adapter, those are pretty bad, plus your computer is kinda weak in the CPU department as well. Since you'd be confined to using that in a "desktop" mode anyways, it makes more sense to save up a little more money and build a Ryzen 2nd gen desktop computer just for Dolphin when they get released in the near future.
But if you're really set on using this express card setup, you probably don't want to go below a 750 Ti, because you're losing performance already with the express card so you want to have a good headroom to start with. Plus its a very power efficient card, so even though you'll need to buy some sort of power supply system, it won't have as big of an impact as a 650. And if you get a 1050 Ti, you might as well just go ahead and build a new desktop.
As for the external express card adapter, those are pretty bad, plus your computer is kinda weak in the CPU department as well. Since you'd be confined to using that in a "desktop" mode anyways, it makes more sense to save up a little more money and build a Ryzen 2nd gen desktop computer just for Dolphin when they get released in the near future.
But if you're really set on using this express card setup, you probably don't want to go below a 750 Ti, because you're losing performance already with the express card so you want to have a good headroom to start with. Plus its a very power efficient card, so even though you'll need to buy some sort of power supply system, it won't have as big of an impact as a 650. And if you get a 1050 Ti, you might as well just go ahead and build a new desktop.