Got a Pre-built Computer I'm looking to upgrade the graphics card on 4 gaming

Best all around card for the money is going to be the GTX 970 or the R9 390. With either one you'll be able to play the newest games on high or ultra with very playable FPS. These cards range around $300-$400.

Sure. Let us know if you have any more questions.

you have lots of options, I did the exact same thing with a Dell and Lenovo tower of the same size. You will have to certainly replace the power supply. I would recommend Corsair 750M they are around 70$ and probably the best power supply for the price. It will last forever and takes up less space using modular wiring.

The GPU is going to need to be small or else you will probably run into issues with it fitting. In both systems I built I had to use shorter length ones. I used MSI gaming series mini ITX for both. I use the r9 270x in my system and the GTX 760 in one I built for a friend. I've also seen some decent single fan models from Gigabyte as well. I tried out multiple dual fan models in both systems and each one fit in very funny and I didn't feel comfortable with it that way.

The processor and ram are both good as is. I put a fan on the outer part of the left wall with the small cut out holes that comes off which blows air into the system. I mounted it with small zip ties and put a fan filter on the outside of it. Not sure if yours has room in this other spot, but I put another fan under the very front of the pc near the hard drive which blows air towards the back of the computer. It runs extremely cool with solid airflow.

The motherboard is fine, it is pci x 3.0. Maybe you can add a Samsung SSD for insanely fast load times. I wouldn't even consider anyone outside of Samsung for an SSD. I've used other ones for customers I built systems for and Samsung is by far the best I've seen. The magician software makes cloning your drive a breeze. If you don't know what your doing very well then cloning a drive and making that new drive the boot up drive can be a bit complicated now that windows changed how you do all of that. It's not over complex, but it's not as simple anymore as a quick change in the bios. The Samsung software will do everything for you 100%. You hit one button and your SSD is cloned with windows on it and becomes the primary drive.

I was just checking newegg and it looks like mini itx size video cards are much more popular now than when I bought mine. This below is probably the best gpu for the price to pair with your processor.

http://www.newegg.com/Product/Product.aspx?Item=N82E16814487093&cm_re=gtx_960-_-14-487-093-_-Product

200$ for that + 79$ (PSU) + 130$ (240gb Samsung ssd).. You can easily get started for under 300$ and then add the SSD whenever you want.

Something like that can easily run all of the newer games coming out @ 1080p 60fps on medium / high settings.