GT520 is weak (IMHO)
Go for something in the line of GT560, GT460 or along the GTx60 line. If you find an old GT460 cheap then go for it.
I have a GT630M (aka GT540 with new name) in my Notebook, and even if it's a little week, it's passable, it can run Skyrim nice with somewhat low settings.
I had an AMD Radeon HD6870 in my old desktop, it ran great but was expensive (not the best card and too expensive for me)
In my older desktop I had an 9800GT, that card was later a little bumped up and renamed to GT250 and after that I don't know what happened to it. That card was great in the aspect of it being cheap as hell and having a great performance (for its price), I think that one performs around the same or better than my GT630M and ran Skyrim pretty good, it's way better than a GT520. The GT250 is an old card that doesn't support DX11, but I'll go with that over a GT520 anytime.

What I'm trying to tell is, if you're going to change your card, look for on that performs nice and at good price. I don't recommend GT520.
I don't know the prices right now, but I would go for a GTx60 (GT460, GT560, whatever) for good performance/price and for a GTx50 (even a GT250 today. But remember, old card, no DX11 support) for good price.

I've been not into gfx cards for some time, so my info may not be the best, you should better look around in google for benchmarks and reviews before deciding.

This post has been edited by sarkwalvein, Sun, August 19th, 2012 at 12:40