Hi mate, if you want to play new games like Diablo 3 on your computer, you have 2 choices for graphics cards :
-Choice one : If you want to play in Low/Medium graphics settings, you should get the NVIDIA GeForce GTS 450 wich is a good card and should run you good in medium settings, but it depends on your screen resolution and the Video memory (1Gb, 2Gb, etc...) and the type of video memory that the card has (DDR 2, DDR 3 , GDDR3, GDDR5, etc...) memory types vary by card constructors (MSI, Gaiward, EVGA, Etc...) I own this card, and am playing some of the biggest hungry games with it even in the maximum settings.
For this card you'll need a minimum 400W power supply or greater, the card is not expensive and offers to low budget gamers the quality and performance that they're looking for, so i strongly recommand you to go get this card.
-Choice two : If you want to play in High/Very High/Ultra settings, then you should get the NVIDIA GeForce GTX 550ti, it's an awesome card that could be your perfect weapon for DirectX 11 games, i recommand you EVGA's GTX 550ti wich has 1GB of GDDR5 video memory.
The card needs a minimum of 400W power supply or greater, also check your computer case size before buying it, because the card size vary by constructer, if you buy one that has a big size and put it in a small computer case, it will have heat problems, wich can lead to lower the performance and sometimes shorten the life of your card.
As the GTS 450, the GTX 550ti is a card for low budget gamers who want performance and quality.
You have a choice between the GTS 450 and the GTX 550ti, they're both great cards.
If you want something else, i recommand you to choose NVIDIA, as i prefer NVIDIA then AMD (ex ATI) when it comes to gaming.
★ The choice from Low Settings to Very High Settings ★
-NVIDIA G Series : Low graphic settings.
-NVIDIA GT Series : Low graphic settings.
-NVIDIA GTS Series : Medium graphic settings.
-NVIDIA GTX Series : Very high graphic settings.
★ Video Memory ★
-Don't get a Graphics card just because you saw that it has 2GB of Video memory and say "Oh ! this card has 2GB of memory ! it will run all the games in great condition for me!" , you can find even a graphics card that has 4GB of video memory that is for 50$ and another card that has 1GB that is for 600$, i'll explain this to you
You see, there is types of video memory like DDR and GDDR, a 1GB GDDR5 card, gives you more quality and performance than a 4GB DDR3 card, you see what i mean ? now that's the most important thing that you need to keep in mind when buying a new graphics card, don't let the store owner tricks you by giving you a 4GB cheap graphics card and telling you that it's very powerful because it has 4GB of video memory...
I hope i helped out,
Sprunk95