As Paul said, the Dark Con rules are pretty much T2k 2nd Ed. rules. Back in the day, GDW made three games sharing the same rules set -
Twilight: 2000 2nd Ed. (including the spin-off
Merc: 2000),
Dark Conspiracy and
Cadillacs & Dinosaurs. Later on, they produced
Traveller: The New Era and started updating their rules set to a D20 form which would then be applied to the previously mentioned games.
The book that copeab is thinking of is the
Proto-Dimensions Sourcebook that has an appendix at the back detailing the changeover to a D20 system in line with T2k 2.2 rules.
However, the rules version are not something to worry about overly much, just substitute the T2k 2.2 rules for the Dark Conspiracy rules and that takes care of everything.
The only thing you have to consider is that Dark Conspiracy doesn't take Fatigue into account and does not do time or travel as four hour blocks (but there is a sidebar in the 1st Ed. Dark Con book explaining about the differences in travel between it and T2k)
The 2nd edition of Dark Con does have D20 rules but if you've got the 1st Ed. main book then the only new thing you're getting in the 2nd Ed. stuff is the rules update and that's it, (there is no "new" content added).
P.S. Here's another resource for a general overview of the game
http://www.waynesbooks.com/DarkConspiracy.html
And just for the sake of curiosity, this is Wayne's Books page for T2k
http://www.waynesbooks.com/Twilight2000.html
If you want to buy any of the material you're better off looking at the PDFs simply because they're cheaper overall. Get them from DriveThruRPG or on CD-ROM from Far Future Enterprises here
http://www.farfuture.net/FFE-CDROMs.html
However, if you ever find a hardcopy of Cadillacs & Dinosaurs, it's probably worth buying as that game had a very small run and doesn't appear to have been made available for PDF sale like Traveller, T2k and Dark Con.