Yep I'll second pretty much everything above, spring guide if it's plastic is a good thing to upgrade going up in spring. But since mentioned yours is metal you shouldn't need to fiddle there at all. Biggest suggestion when tearing apart the box is take your time and make sure not to mix up where the shims on each gear go(pending G&G's are shimmed normally). Since that can screw up a new guy opening a box. I wouldn't worry about improving the shimming at this point especially being new since it can be a bit more advanced.
I'd also say stick with a max around 400fps because that won't limit you as much field wise compared to going over 400, and the range advantage possible with that 20fps isn't enough to really matter, we're talking maybe another couple yards max. And that's with everything else tuned and working perfectly.
On range and accuracy, barrel quality effects accuracy, but also bucking quality as well as consistency of the air seal through the system and consistency of the application of pressure on the hopup nub/bucking. So to start just make sure to watch some youtube videos where guys show you how to test compression at the piston/cylinder/cylinder head/nozzle area. If you're not running an oring nozzle(not sure on g&g) then it would be a good idea to slap one of those in there as well the SHS version has been good to me and is cheap. Once you're sure your compression is good on the gearbox side put it all back together.
Then you can work on the hopup end. I have used a G&G hopup and found it decent enough as well as their typical green bucking. So you're probably not going to see huge gains on that end at least from what I've seen. So I'd hold on that for now and see how it shoots. If you want to step it up you can go with one of the rotary hopup units, I like the Madbull rotary hopup unit and matching bucking. I've had great luck with those, though getting them setup can be a pain. But one of the others may work fine as well. I do like madbull buckings, though again it's sort of preference there. You can consider flat hopping or rhopping, both can improve the consistency of the application of the hopup. But neither are really "necessary" either especially I'd say at the sub 400 fps ranges. I generally start really tuning that stuff for longer range and higher FPS builds.
So short version yes, if you're careful you should be able to just slap in a spring. Do keep in mind you probably will not see a massive difference in range etc. 320-400 isn't a massive jump. You will be able to run slightly heavier BB's more effectively which will net more improvements than most other mods. Consider dropping in the spring and going to a 0.25 minimum(off shoot, use 0.25g in virtually anything or heavier). You can jump to a 0.28g-0.3g BB's maybe even a touch further though keep in mind the price will jump for the consistency of the heavier round. As you might run into more feeding issues from some mags that are picky, etc.
Everything is a give and take, also consider if this is your only gun that mods like upping FPS even with a spring change will shorten the life of other components like piston, gears etc. How much depends on how well the gun is put together by the factory as well as you. But it's sort of like driving cars, if you've got a hot rod, you don't want to use it to get back and forth to work. You need a daily driver so to speak and a hot rod;) At least if you want to insure your gun isn't going to go down during a game and leave you without the ability to keep playing. Most guys have had a gun go down in a manner that isn't easily fixed on the field at some point and while it sucks, it's worse if it's your only gun;)
Take care!
Luke
