Is the EVGA GeForce GTS 450 Superclocked 1024 MB GDDR5 PCI Express 2.0 2DVI/Mini-HDMI SLI Ready Graphics Card, 01G-P3-1452-TR be a good upgrade from the NVIDIA GEFORCE GT 520 for playing video games including Skyrim???
The GTS 450 is an upgrade. It's an older mainstream card. In general, it should give you good performance at 1600x900 resolution with high graphics quality and decent performance at 1920x1080. Performance varies by game so you may need to drop graphics quality down to medium. For example, at 1600x900 with high graphics quality it will probably choke when playing Metro 2033, but would be okay with Far Cry 2.
Whilst the GTS 450 would be capable of playing skyrim (my laptop with a gt330m could play it at reasonable settings), unless you are getting a very good deal on that particular card, I'd look at something more recent.
The 7750/7770 or the GTX 650 (ti) are current gen cards that are in the same price range as the GTS 450, but have better performance and much lower power draw.
If you give us a price, your current system spec (include PSU) and a website(s) that you prefer to buy from we'll be able to help you much more.
The GTS 450 is an upgrade for the GT 520 though? I already have it and I have the 520 in my system now. What does all the "Superclocked" mumbo jumbo mean? Anything significant? The GT 520 has problems with Skyrim (i.e. system crashes and mouse (cursor) control problems).
If you have it and a capable PSU, then you should unistall the graphics drivers, swap the cards, install the correct drivers for the GTS 450 and go!
Superclocked means that company who made the card overclocked it for it's customers resulting in better performance at the cost of power consumption and heat, but the overclocks tend to be quite mild so you shouldn't worry to much.