Hi
Hello!
I have some questions and I hope some of you can help me.
I'll try to answer to the best of my ability.
What kind of paper has the Nintendo DS games?
If I had to guess, something similar to magazine paper.
What kind of paper is the best for printing covers? (Type, thickness, etc)
From your home printer, I'd say 8.5 x 14 (Legal Size) if you're from the US, or A4 for international. You want semi-glossy paper. Don't worry about thickness with the semi-gloss, I don't think it varies that much. If you're printing on Legal, you may have trouble finding it in semi-gloss, so keep that in mind (A4 can easily be found in semi-gloss)
I read the FAQ but I still need some experiences and thoughts about this.
I want to do some NES covers using pictures of the GameFAQs site, but some of them have very low quality, my question is: What is the lowest resolution to do a nice UGC cover?
Here is an example:
Motor City Patrol – Front cover
http://www.gamefaqs.com/nes/587473-motor-city-patrol/images/box-46957
The front cover resolution is 482px × 691px, Can I do a good looking UGC cover with that picture?
482 x 691 is the image size, not the resolution. Covers tend to be rather large in terms of image size, and the resolution is at minimum 300dpi (dots per inch), though some are 600dpi. If you tried to print out that image at the dimensions we set for our covers, the entire thing will look blurry. You can test it out yourself. The front of a Universal Game Cover (the case we store NES games in) is 1536px wide by 2100px tall. Try stretching that Motor City Patrol to those dimensions in Paint and you'll see what it'll look like when printed.
Note:
Sorry if this post does not belong here; I could not post in the FAQ section.
Thank you 
You're welcome. I hope this helped.