Hi and welcome to RFGen!
There are all kinds of variations out there, some scans of variations are in the database under "Extra Media" in the respective gamepages (i.e.
http://www.rfgeneration.c...l?ID=U-027-S-02790-A&).
For NES games, the most notable variations are different seals (as you've already mentioned), and also different part numbers. Rereleases may have a -# (i.e. -1 or -2, etc) added to the part number of the box, manual, and/or cartridge label. Lastly, there are also various other kinds of variations:
*Licensed and unlicensed versions of three Tengen games (Gauntlet, RBI Baseball, and Pac-Man).
*Square and cut-corner AVE (American Video Entertainment) cartridges; some have the top left corner cut off in the design, some do not.
*Color of cartridges; games from Color Dreams, Bunch Games, and Wisdom Tree come in either or both black and blue cartridges, also with different back labels (embossed, grey, blue, black).
*CAN games; some games were released in Canada with the -CAN code instead of -USA. I'm pretty sure that only the first party titles from Nintendo have such releases.
*5-screw/3-screw games; at the end of 1987 (if I'm not mistaken), Nintendo remodeled the cartridges to only use 3 screws (and two clips on the top) rather than 5 screws. Some games were released in both formats, and some versions are pretty hard to find. 5-screw carts can be identified by the flatter top, as they are missing the clips seen on the left and right side of the top of a NES cartridge.
Well, those should be the major variations, although I may have missed some. There's also tons of small variations, but they're just too small to generalize.
Good luck with your hunt and stick around!
Feel free to introduce yourself in this thread:
http://www.rfgeneration.c...um/index.php?topic=2931.0