Duh... think I found your system, it should be the ASUS Essentio Desktop M51AD B08 i7
If you told the Best Buy guy that you were looking for a gaming computer he either is an ******* and didn't care or he just didn't know.
- A way cheaper i5 instead of the i7 would have been enough for any game except perhaps a few hardcore simulations, but for them you'd need a better video card anyway
- 8 GB of RAM would have been enough for gaming, 12 GB don't make any sense anyway as for dual channel (a bit faster) you need 2x the same RAM which isn't possible with 12 GB as there's no 6 GB ones
- the GTX 750 1GB is crap for any gaming, not only for modded games, just try playing any next gen game on medium settings and you'll fail
- the case is probably worth 30 bucks (retail) and crap, just like any OEM case, cooling the new video card could perhaps be an issue, ASUS doesn't provide any info about fans & cooling
- you only got a HDD while you would profit from a SSD drive so ******* much but I guess random people don't know about SSDs and 2TB of storage sells better than 256 or 512 GB
- the PSU isn't even described on the ASUS page, all they give me is 350/500W (peak) which could be too weak for a good video card, especially since it seems to be no name
Still got the chance to return the thing?
Anyway, never ever buy a ("gaming") computer in a random store again. I can tell as I worked in an actual computer store myself for three years and literally ALL those computers I got (I was the service guy) were crap, had super cheap OEM mainboards and PSUs, had underwhelming video cards and PSUs so weak they wouldn't stand a better card and were brought to us as the service departments of these stores usually don't know what they're doing. Many of them also had mainboards and cases that didn't meet common standards which makes replacing parts after the warranty a pain in the ***. DELL was the worst when it came to the latter.
If you got the chance to get rid of that thing with no or little loss of money I would do it and built a custom machine for the same prize that is WAY better for gaming and WAY more future proof.
If not, you probably have to replace your PSU along with your video card which will cost you at least another 50 bucks. Or you use the default one until it dies, in most cases no other components are damaged and when its too weak for your card you usually see your computer turning off during gaming or having horrible performance (making your new video card useless). ^^