there are also a few modern wannabe X-Coms that aren't horrible if you don't feel like fighting with dosbox and moslo to get old games running at a playable framerate (as opposed to ludicrous speed).
the UFO:After X series was good. (afterlight and aftermath - pretty sure there's a 3rd).
this is a link to a remake using one of the quake engines as well. it's enough to give you an idea of what the original x-com would be if you don't want to pretend like it's '96 again just yet.
http://ufoai.sourceforge.net/
Terra Nova - Strike Force Centauri is another title that i think got unjustly overlooked. i think it suffered from the same problems as the original System Shock and Ultima:Underworld titles. they needed a really high end system when they first came out and were overshadowed by newer titles by the time most people had machines that could handle them.
Syndicate and Syndicate Wars are another pair of great old school titles, then again ANYTHING by Bullfrog games = AWESOME. speaking of which. DUNGEON KEEPER 2.