Conspiracy X is published by Eden Studios, the same folks who made All Flesh Must Be Eaten and the Buffy RPG. It's a good game, and the character creation system is cool. But I found the system to be too clunky for me. I'm not a d20 fan, but I'd much rather use d20 than the Con:X system. Also, the setting and plot aren't that original. The Greys, government conspiracies, etc. are straight out of the X-Files. Delta Green and Dark*Matter did a better job in taking old material and making it their own.
I wish I knew more about Dark*Matter, but I only read a couple of things here and there. Still, I liked what I saw and I wish they could've done a lot more. Who knows, maybe they'll do a d20 conversion...But the Alternity system works fine, I think.
My suggestion: Find someone who knows the games well, make a character and you choose. Me, I'm going with Delta Green. It's not just good game material, but reading material in of itself.