1. Metal Gear Solid 2 is very highly rated on Gamespot. Typical question: If I haven't played a MG game since MG2 on the NES, will I be lost if I pick up with Part 2?
Even if you played MGS for the PSX you'd probably be lost :lol: It's a pretty confusing storyline, but I don't think you'd lose enjoyment if you skipped the original.
2. MGS2 was redone for the Xbox and subtitled substance. If I'm going to pick one up, which should I get?
My playthrough of MGS2 was with the Xbox version of Substance and it worked out very well. No complaints. I think it just adds little missions you can playthrough if you wish on the title screen, a mode to fight all the bosses, the ability to collect dogtags, etc. I see no reason to pick up the original if Substance is out.
FWI, there is a PS2 version of Substance, but it's somewhat rare (and I just unloaded my copy of that for $30 on eBay).
And Kevin Grey seems to be the MGS-expert around these parts, so I'd get a reply from him to trump the rest of the replies

3. Is MGS just a sneaking game like Splinter Cell?
It is so much more than just a sneaking game...sneaking does take up a large portion of it, though. You can probably run & gun through most of the game, I'd say. It would be somewhat harder, but possible. At its core, it's a sneaking game, but with the awesome boss battles and whatnot, it isn't strictly sneak, sneak, sneak 100% of the time.
Also, if you DO want to play through the series chronologically, I'd pick up the Twin Snakes (only for GameCube if you own one) and play through that first. I've tried playing the original PSOne game, and I just love the way Twin Snakes plays.