Did a quick look up and it doesn't look like their is a Continental PC at IAD, although there is a NW WorldClub you should be able to use in the B concourse.

The concourses are connected by 'mobile lounges', basically funky buses that run approximately 5 to 10 minutes. Depending on what concourse you arrive or depart it will take 5 to 20 minutes each way.

Their are two RCC in concourse C and one in concourse D. It's actually the same building. I'd recommend the one near gate C17 - it's close to the mobile lounge gates and fairly good sized, although it will be crowded during peak travel times.

Here's a map
http://www.united.com/page/article/0,1360,1124,00.html

Unless you have a lot of time to kill, I'd pass on the RCC passes, unless you can get a day pass at a good deal. You might want to check if the day pass allows guests.

Cheers,
Jeff