An X-valve will give you protection from short stroking at high rates of fire. If you're like me, you probably get on the trigger when excited, and then get a half released trigger cycle which causes a short stroke and a cough. It doesn't cause problems, but it is annoying. The X-valve will help prevent that because of its ability to recharge in much less time.
The X-valve comes with the level 10 bolt. If you don't get the X-valve, at least get the level 10 bolt for your classic if you don't already have it. It is the best upgrade for any mag.
The ULE body is also a nice upgrade. It gets rid of the nubbins and the barrel wobble that goes along with a twist lock barrel. The cocker threads are the most common barrel threads, so it opens up a lot of barrel possibilities. Make sure you keep a spare detent on hand in case the one in the body gets damaged. The detents are a common angel thread.
The clamping feedneck is a nice upgrade on the ULE body. It is a standard Angel thread which a lot of markers use. It gives you lots of options.
Any rail will work. You can use the stock AM/MM rail until you find one you absolutely must have. If you do upgrade, try to find one that is milled from a retro rail so that it uses the screw in sear pin and retro sear.
There are lots of great grip frames for automags now. Check out some of the dealers that sell here on AO. They have some really nice frames. The triggers are really stable and smooth.
You don't need a regulator. A regulator will hurt your performance. There is already a regulator on the bottle itself and one in the mag valve.
To get an X-valve to be reactive, you just need to adjust some of the parameters. The easiest one is to get a bottle with an adjustable output pressure that can reach 1100 psi. The next one is to get some ULT shims for your retro on-off assembly. You can add up to 4 shims to increase your reactivity. You can start with a shorter on-off pin if the 4 shims don't give you enough reactivity.
Except for the Automag in front, its usually the man behind the equipment that counts.