Great first cover! You did a very good job with the cover. 
Suggestions/Questions:
1. Mario Party was made by Hudson?
2. The front image is a little grainy.
- If you are using photoshop then select front image layer and go to Filter => Noise => Despeckle
- Don't do this more than 3 times or the image will get super blurry though.
3. I think the cover would benefit from having the spine logo centered (lowered from where its at).
Thats all that I am seeing so far. Good job introducing yourself to the site!
I've updated the cover with despeckle x2. Thanks for the tip, never used that before.
I kept the logo on the spine at the top to try and replecate the retail box but if you really would rather have a centred version it would only take 1 minute.
I agree with the die comments although I've never actually noticed it before. It's odd that they chose to place a die there. I guess they didn't think they would go on to make 8 Mario Party games!
Here's my PAL version:
http://www.box.net/shared/2x7ml0fqnhh4igvkxv09Being British I understand why people want these. I haven't actually started casing my N64 games yet so I haven't decided whether to use NTSC or PAL covers yet.
Thanks for the praise and tips. it's really encouraging. Up next? Mario Party 2 of course!