Actually I also tried to have some info about useful add-ons, but I got rather confused, than clear after googleing. I found too much:

Scenery: Ultimate Terrain X, Flight Enviroment X, Ground Enviroment X, REX 2.0 + Overdrive, Active Sky (full or lite), FS Global

Traffic: Traffic X, , Air Traffic FX, Ultimate traffic 2

I really dunno which to choose...

Its easy to get confussed, since there are so many addons to choose from.

Unfortunatly, you will probably want to get most , if not all the addons you referenced above. so it depends on your budget.

I would start with GEX & UTX, they make the default scenery much better. REX or Active Sky 2012 will make your skyies much more vibrant and detailed. I had both, but sided with Active Sky 2012. http://www.hifitechinc.com/products/active-sky-2012 http://www.realenvironmentxtreme.com/

For traffic, either UT2 or My traffic are good choices (only choices really). I found that My Traffic populated the airports better, but UT2 had nicer looking models. If you plan on ever going to Prepar3d in the future, get My Traffic, because you can get the P3d version of my traffic for a lot cheaper as a previous customer.
http://www.fspilotshop.com/product_info....CFUio4AodDAsB3g

http://www.flight1.com/products.asp?product=ut2

Another must have for me is the Reality XP GPS to replace the stock GPS. I would only get this if your really into simulating as close to the real thing as possible. http://www.reality-xp.com/flightsim/gns430/

"Shade" is nice and adds a sort of enb type of look to FSX and is configurable. Its also cheap.
http://shadeforfsx.blogspot.com/

If you have 2 monitors and want to use the second for a moving map? "Plan-g" is real nice and its free! or Flightsim Commander for a payware version. http://www.tasoftware.co.uk/planG.htm http://www.fscommander.com/

For aircraft, you cant go wrong with anything from A2A. Carenado also makes nice stuff. If you into complexity and large airliners, than PMDG or LVD are the best around. http://a2asimulations.com/ http://www.carenado.com/CarSite/Portal/index.php

The best Large airport scenery comes from either Fly Tampa or FS dreamteam.
http://www.flytampa.org/

http://www.fsdreamteam.com/

The best GA airport scenery or bush flying scenery is from Orbx.
http://fullterrain.com/

If you fly around europe, check out Aerosoft. They have a lot of europe scenery and airports as well as some major US cities covered very well. Manhattan x is my favorite (because I live there lol).
http://www.aerosoft.com/cgi-local/us/iboshop.cgi?show270

If you want moving trucks and people to service your planes, There is GSX and AES. GSX is from FSDREAMTEAM (see link above) and AES is from Aerosoft (see link above.)

Last but not least is Gamebooster, A free utility that will free up resources for running games, helps reduce annoying stutters.
http://www.iobit.com/gamebooster.html

Hope this helps a little.

Rob