Get it from GOG, download the Echo Patch and make sure to play the first expansion too (which is included), called Extraction Point. I think it's even better than the base game actually. Definitely DON'T play the second expansion, and in fact probably just pretend the series ended after Extraction Point.

The game absolutely holds up, still plays like a dream. One of the best FPSes of all time for sure, great combat with good AI and fantastic gunplay from most of the arsenal (apart from a strangely anemic assault rifle). Also looks remarkably good for its age - some of the distortion effects especially like bullet traces during Bullet Time or shockwave ripples from explosions look surprisingly good for a 2005 release.

You can also jump through some extra hoops to make EAX audio work but it's a bit of a headache.