UFO:Aftershock is TBA in the middle of October... early previews state its more complex but some niggles from its predecessor still plague it... although there isnt anything else in the market it could be fun.
I also remember playing X-Com and is one of my favorite old games. If you're looking for a similar game with the same addictive turn-based gameplay, check out Silent Storm. It doesn't have anything to do with aliens, it's based in WW2, but it's as close as you can get to a good turn-based strategy game. They have it over at Take2 for about R250 (ex. shipping). Check it out!
I want to echo what tibby.dude said. Guys, forget about getting patches etc. Just download and install DOSbox from http://dosbox.sourceforge.net
It's brilliant! It emulates a 486 on a powerful Athlon or P4. The old stuff just works. And it emulates Ad Lib, General Midi MT32 etc for the really old games. (how many people has the awesome MT32 back then?)
Just one thing though, old games that use the DOS4G/w DOS extender (DOOM, DOOM2, Duke Nuken 3D) might run too slowly unless you have a top-end rig. But XCOM1/UFO:Enemy Unknown, XCOM2-TFTD, StarControl2, Monkey Island etc. run nicely on my rig.
For Lucasarts games that use the Scumm (Scripting something something) engine - originally used for Maniac Mansion IIRC - the best thing to use is ScummVM (http://www.scummvm.org/). Of course then you need special binary files of those games, but you can find those on a few abandonware sites. One of the only free ones you can easily get is Beneath a Steel Sky, which was also released on one of the NAG CD's together with the ScummVM program.
This is detracting a bit from the original post, but hey since people mentioned Monkey Island and DOSBOX, thought I'd add in this bit
R45! Yeah I can see why that would be worth it. Took "20" hours to get 281 Mb from a site in the UK. It should be done by the time I get home. All the effort is much appreciated tho