Cap'n Noodle -
ProFlight 2000 is an add-on from a company called AETI (http://www.simpilot.com/)
Basically you create "adventures" for yourself which contain ATC support. So for example, you'd file a VFR or IFR (more interesting if it is IFR, frankly) and then compile the adventure. Then when you start FS2K, you can load the adventure and Bob's your uncle.
Your side of the conversation is activated via keyboard commands, although you can use a third party voice recognition product for gamers (which I had little success with).
It certainly adds to the overall experience, although you do need to go through the planning steps, which adds to the overall amount of time you spend behind your PC. Still, I don't suppose most of us jump into a real airplane and go, so perhaps that's what is meant by "as real as it gets"
The two shortcomings are: (a) lack of flexibility, since the adventure is essentially scripted, (b) FAA terminology and procedures throughout even if you're flying in Australia (although in this version they do have numerous voice sets that are used automatically, so you'd hear a Qantas pilot speaking with an Australian accent). The other minor but highly entertaining point is that it will happily chuck big airplanes into small airports (e.g. "Speedbird 1234 cleared for take-off" when you know that the runway is 2500ft long...)
If really want to be a sad sod (like me), you can add Ultimate Airlines, which allows you to use "real" flight codes and plans for major airlines and routes. So you can be UA954 on a translatlantic route in a B777 or whatever. As I recall, ProFlight on its own has a library of DPs and STARs, so you could potentially use it as an aid for practicing some IR stuff.
Whilst on the subject of heavies, Phoenix Simulations (www.phoenix-simulations.co.uk) do an excellent replacement Boeing 777 and a Boeing 747-400 for FS2K. Precision Manuals (www.precisionmanuals.com) do a 757, 767 and 777 for Fly!2K. None of these are super expensive and they seem to patch them regularly and generally keep them up to date.
Fly has ATC built into the product, as does X-Plane. Again, they seem to be all FAA-based though, so I'm not sure how useful they would be to you.
Microsoft Flight Simulator 2002 is supposed to have embedded ATC in the product, and is supposed to be pretty good for VFR in that you can get flight following and traffic advisories, and it is all dynamic (i.e. not implemented as an adventure, and not scripted).
Hope that helps.
David.