Wait until your nylon bushings wear out before changing them. 7mm bearings will do a great job. Don't use a battery higher than 9.6V...the ICS wirinig isn't designed for high voltage batteries...they'll corode quickly.

bushings > bearings.  And good luck getting 7mm anything into the gearbox -- shimming is a nightmare with a distorted shell or maligned bushings.

A stock ICS M4 will handle a M120 spring--grease the spring really well with Sta-Lube white lilthium grease (you can get this at any auto parts store--and its much better than the Systema or any airsoft brand, much cheaper, and you get more than you'll ever use). It takes only about 5 minutes to change the spring.

Adding new grease to the system without properly cleaning and relubricating everything else is not a good idea.  Be smart when you upgrade to clean everything very well, and re-grease appropriately.

So: I'm wanting to upgrade my ICS M4, all I really want to do is increase the FPS cause it's already accurate. I'm not sure which spring I should get though, or if I should just buy a kit. If I get a new spring I know'll I'll need new bushings but I need advice on which ones I should get. Or if I should bother upgrading my gat at all.

Honestly, just buy a complete upper gearbox  half.  It's usefull having more than one around, especially if you need to quickly change to stock FPS for a CQB. This is what you need.  Running anything over stock WILL wear the nylon bushings out quicker (as discussed above).  Replace them all at once, when the first one wears out.

hell i havnt ever cleaned mine but thats because I'm just to Ghetto to afford a brush and it is still a chugginafter 4 years.

that's just plain gross

Myself and 5 others (ages 18-21) are in the far east side of Tucson.