What tier of video cards should I look at? Should I go for the top tier just to stay ahead of the curve?
No. You definitely don't want to shell out for the latest & greatest if you are cheap -- those cards cost as much as a half decent work PC.

There are a few “sweet spots” -- just upwards of $100,- mark is usually last generation's pretty good to very good, this is essentially the bang for the buck segment. Round about the $100 is essentially the place where reasonably up to date manufacturing meets price pressure due to newer models which means that it is essentially the stuff you used to pay $250,- for only a year ago.

Another place is about the $250 mark, which is usually where the respectable middle tier cards are priced at. Past that pricing gets steadily more extravagant for diminishing returns, especially when you factor in power consumption. So if you want to buy now and be able to play games well into the next 5 years or so, then the $250 spot is where it is at.

Below $100 you tend to end up with the ancient/outdated or the stock that failed to shift, or with the shortcuts to make the price point. (Like replacing GDDR5 with GDDR3 in a reference design.)

In your case I would go for the “about $100” segment for cards. Pretty much anything in there should outperform your current 520 by a wide margin, at roughly similar or better power levels (from the newer manufacturing processes).