JG Bar -10 upgrade choices

First, you should always invest in accuracy and consistency upgrades. These include barrel, hopup, as well as checking all your seals.

However, to answer your question, once you do the accuracy upgrades, use the polarstar stuff.

Also, try this thread.
http://www.airsoftsniperforum.com/phpBB/phpBB3/viewtopic.php?t=265

Typically you do not have to change the cylinder head, however one of the first things you should change is your sear set, and then work from there. There is nothing worse than having a cylinder upgraded and then your trigger group exploding or just not working anymore because you over powered the stock sears.

Here are a few good links to get started with to familiarize yourself with the gun (since its a VSR clone)
http://www.airsoftcanada.com/showthread.php?t=37455

I would typically try to stay away from Airsoft Forum, they dont really get into the actual art of working on your guns, typically they just want to dump money into them to make them shoot hot and then wonder why things keep breaking.
Finally, I would suggest you re-post, or have this moved to the VSR forum, you'll probably have more replies there instead of in the "others" section.

Thanks both of you. I think I will do what both of you said, in order. Do all the accuracy upgrades, then move on to the more of the internal stuff. Thanks again =)

Accuracy is internal stuff, you will change the inner barrel, put in spacers, stuff like that...

I know that. I was meaning though, that after I did accuracy stuff, I would move onto bigger things. But hopefully I wont have to upgrade anything, because I might be buying a fully modded Bar 10

Ah, right.