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.
Trigger Frames and Bodies -
The Top of the line mechanical frames for mags are the Intelliframe and the Y-grip. Both have intellifeed abilities as well as a nice crisp pull, built by airgun for airgun. There is also the Z-grip which is no longer sold, there are a few floating around if you want one. The ULE(Ultra Light Engineering) bodies are also 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.
Bolt and Trigger Kits -
The only bolt kit worth getting is the level 10(see section Level 10). You should also look into the ULT trigger kit(see section ULE Trigger Kit)
Valves -
The Xvalve is the way to go, comes with level 10 installed and ULT ready.
RT Upgrade Program -
Turn your RT into an RTP so it can accept the latest upgrades, Click Here to find out how.