I think you’ve got two ways to play this;-
1), Buy the GPU that you were thinking of getting (check you have an appropriate PSU to power it) and pop it into your existing system, if you can achieve the desired results in the games you want to play job done (your board supports PCIe 3 so should be good to go, although if you also have the m.2 slot populated it might reduce the capacity for the PCIe lane... I’m a little out my depth in this regard and don’t know enough about your board to know if this is something you’ll need to consider).
2), If you get the new GPU but find that your system is holding you back on the games you want to play then it’s going to mean going down the upgrade route, this can be cheap or expensive depending what you need to actually upgrade but if you end up in this situation pop back here and give us a more detailed breakdown of your existing setup, your budget for the upgrade and a list of the games you are looking to play, then someone will be able to advise the best upgrade route and how to plan your budget/build to get the best results you can
As far as what GPU, personally if you are planning to potentially take it forward into a future build I’d say go for the best that you can currently afford, if you’ve got £300 to spend that will put you in new 5600XT // 1660 Super or Ti // 2060 tier I’d imagine, although prices and availability have gone a bit crazy this last year (there might be a 3000 series nVidia in this price bracket but I can’t find any), another option would be a used GPU that might extend your reach to a model or two higher from a previous generation but I’ve only had bad experiences with used PC components so if it were me I’d buy new so you can return should anything go wrong (I had the AIO fail on a used 2080Ti... you can imagine my frustration considering it was a £1300 GPU).
