The Obvious -
The best upgrades you can get for any set-up is a good air system and a good barrel kit. If your in the market for an adjustable air tank, I personally recommend the AGD Flatline system. If your looking for a screw-in tank, I recommend a Crossfire tank. As far as barrel kits go, I love the Custom Products CP Pro Kit.
Bodies (Emag) -
The ULE(Ultra Light Engineering) bodies are top of the line(54% lighter that the stainless bodies), available in 8 colors both in vertfeed and warpfeed. They are accept cocker threaded barrels, AGD/WDP threaded detents, and AGD/WDP threaded feednecks. Yes, they come with a detent and feedneck.
Valves and Bolts -
Newer Emags and all Xmags have Xvalves(w/ level 10 installed). If you have an Emag that doesnt have an Xvalve or level 10, I recommend looking into it.
ULE Parts (Emag) -
To completely convert your Emag into a ULE Emag you will need to get your hands on a ULE body, Xvalve, ULE Rail, CnC battery pack, and ULE Trigger frame. All these parts(except the ULE Trigger frame) are available through your local dealer or through Airgun's online store. To get your Trigger frame turned into a ULE frame, goto www.automags.org/forums and ask who can get it done.