The cable you have is likely fine. If you want to try something rock solid, get RG6 coaxial cable and run that to your projector then terminate it with RCA connections. You can make it exactly the length you need and it makes everything pretty.
If you are using ANY keystone correction, turn it off and put the projector in the correct location to avoid keystone correction.
I am not overly impressed with the Dell which is quite clearly a business projector, which means if you got it for your home theater, you may be disappointed by it's lack of high quality image processors internally to deal with motion and color the way a home theater projector from a company who has been producing them for a while would. But, you should still be pretty happy with the image.
Now, if you send DVD to the projector, it will look decent, but the projector is not optimized for wide screen DVD's and you will have a loss in resolution on any widescreen material. But, with good source material, you will almost always have a better image. So, HDTV should give you the best image, followed by DVD, finally, all other TV then VCR. You likely won't be happy with TV/VCR quality images.
Monster cable is overpriced for what you get and RG-6 will do an excellent job if you want to try switching cables, but in my opinion, 95% of what you see on screen begins with a high quality image going to a good projector. The wiring may marginally improve things, but if you aren't happy now, the wire will not do it for you.