Best thing to do is to open up your case, take a ruler or tape measure and measure the amount of space where the card would sit (usually the first PCIe 2.0 or 3.0 x16 slot). Measure from the back of the case and all the way to the drive bays and also measure from the motherboard slot to the case cover. If you have 10.2 inches lengthwise, two empty expansion slots on the case (at least 1.5 inches thick) and enough height to clear the side panel of the case (at least 5.2 inches) then the Asus GTX660-DC2O-2GD5 will fit. http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Almost all Asus GTX 660's are the same PCB and heat sink/ fan so they are all about the same size.
I suggest you contact Asus directly. This desktop model has a 350W PSU and can come with a 6.5" GPU, such as HD8350. However, most 660s are around 9.5" That's much longer then the one you might have. Also, most systems with a 660 require a minimum of 430W. You might get by with 400W, but 350W seems too low.
Have you thought about removing the components and putting them into a bigger case with a larger PSU?
I just opened it up and it looks fairly big ni to be honest inside
It might look big enough, but remember that you need at least 10.5 inches. Look at the pic of the Asus GTX 660 and then look at your empty PCIe slot. You are going to need the equivalent of at least 3 of those slots long. Look how small it looks on the card in the pic. Those are 90mm fans on that big girl. Then you need to make sure that those heat pipes bulging out the top will clear your side panel. Eyeballing it on a case that doesn't look like a full ATX isn't good enough. You need to take some measurements to be sure or you will end up buying a new case or returning the card.
Do u think I could just get a bigger power supply 4 it
also if your pc is not neat inside, like wires and cables are everywhere, make sure to tidy that up. i had a hard time getting my 670 in my old mid tower case, it had about 2cm of room left in length and those 2cm were taken up by two large cables that attatch to the motherboard
That Asus case you have looks really nice. I hope you can fit a new PSU and GPU in there. If you are going to reuse the case make sure the new PSU has the same dimensions as the 350W one so it'll fit in the case.