Both cards I suggested you are standard entry cards for gaming.
No one of those are powerful, if you want to know what a powerful card is like or costs, look for a GTX 690.
Anyway, the thing here is that GT520 is weak, really.
I don't think you will get noticeable better results with a GT520 than with a 8400GS. If you're gonna get a GT520, better stay with your actual card.
Look for Skyrim benchmarks on both of these cards if you don't trust me.

Anyway, both (used) cards I suggested are below 120 dollars.
If you want something not used that will work great for that price, then look for a GT250 as I told you before.

Don't look at specs, look at benchmarks.

PS: Trust me when I say this, don't go below a GT250. It's better to keep your actual card than to go for anything below a GT250.
Remember that nVidia numbering system is like (Family)(Performance Grade), and GT520 is Family 5, Performance Grade 20 (the lowest performance grade)
It sucks.

GT 440 is better, GT250 is better...
and 8400GS (old numbering), even being a really old family, is one step over the lowest performance grade (for its time).
GT520 is better than the old 8400GS, but not that much.

PS2: I've just looked, and here in Germany you can find a GT440 new for 66€.
Not a card that I would recommend, but get that over a GT520.
Look for benchmarks in google, look for tests running Skyrim. Don't look a specs, those don't really tell what you want to know (fps, performance in games, etc.)

This post has been edited by sarkwalvein, Sun, August 19th, 2012 at 16:53