http://airsoftcanada.com/showthread.php?t=2738 Read through this thread.
As for needed upgrades, like I said all you really need is bushings, pdi150% spring, tightbore and ball bearing spring guide, everything else can be left stock if you don't want to spent money on aftermarket parts. Those upgrades will bring your gun to around 400FPS. It might go over 400 a little bit since you will have longer barrel (my AK with above upgrades shoots at 390-400fps with .2s) I've also noticed that stock gears, piston, cylinder and piston head work just great, I actually had problems with aftermarket pistons, switched back to stock TM parts and my gun is working without any problems ever since. If you don't go over pdo150% with 8.4 batt stock piston, piston head and gears will handle it just fine.
I used to own a gun which had 99% Systema internals and reinforced mechbox. My current "mostly TM" inside gun works as good as fully upgraded one.