The closest you can get to what Sam actually does -- Navy Seals (specificly DevGru), Army Delta Force, CIA Special Activities Devision. Wikipedia/ google can give you more info at a faster pace than I can.
As for being a single guy who sneaks into places unknown, does very dangerous things in the dark and dissapears? It is pretty unrealistic. Sending only one man is a government liability (especially if the mission is sensitive or important) and covert actions usually require the support of team members (these days, not even snipers can work alone).
Clandestine operations (hiding in plain sight while under-cover, rather than the shadows) can be undertaken by lone individuals (often it has to be this way) both here at home (talk to the FBI) and abroad (talk to the CIA). While the work can be dangerous, it isn't exactly what Sam Fisher does, by any stretch. If you are interested, both the FBI and CIA offer competitive scholarships for college students looking to serve their country and pay for college at the same time. You can find more info at their respective websites.
As for the NSA? They are our nation's code crackers. They handle signal intelligence -- not covert ops. While challenging, rewarding and very secretive it is largely a collection of desk jobs.
Long story short: Being a 250lbs guy, running around with 50lbs of equipment in the dark while being completely silent, hacking computers, killing terrorists, and performing acrobatics that would make most olympians jealous is the stuff of bad spy novels and good video games -- not a basis for an actual career.
Edit: not too sure if this post belongs in this forum though, as it caters specificly to in-game help with the SC games. You might have better luck in an off-topic forum. :-)
Who do you think Woody Mister is? The job he describes in the DA interview seems pretty close to what Sam does. He really didn't say much of anything specific though.
