This is a popular fantasy for many men and couples, but it depends on the couple. Just because your wife can sexually describe a scenario to you during sex, doesn't mean she desires to bring another man into the bedroom. Also, the reality of it is much different than the fantasy. I'm not trying to piss on your parade here, but I've seen lots of couples dive right in and experience some real difficulties. There's a process to maybe making this a reality and the best advice I can give you is to obtain and read thoroughly, The Threesome Handbook by Vicky Vantoch
http://www.allsexreviews.com/threesome-handbook.html
In there she tells you how to go about bringing up the topic to your partner, questions for both of you to ask yourselves and each other to find out if this is something you should continue to explore, deciding on finding a third person, through the whole thing! It is THE book on Threesomes.
Also, Nina Hartley's Guide to Threesomes 2 Guys and a Girl will explicitly show you the possibilities that exist and the trouble spots you might encounter. Plus Nina's always provides an Q&A section, providing even more information for you. It's a great DVD to watch together. Once you see this fantasy acted out, it may help you both to decide if it's right for you. While this DVD is graphic in nature and does contain pornographic scenes, it will show you both exactly what you're in for when bringing another person into the bedroom.
If you're wife isn't open to exploring any of these resources, then I wouldn't advise going any further. If a couple can't talk and learn about this, they do not have the chops to see it through successfully. The key to being successful with additional partners in the bedroom is being able to talk about it all without getting pissy with each other, really talking and really listening.
If you have any further questions, don't hesitate to ask.
Ann