What most influenciates in OS performance ?

Of the 3 items listed the 'slowest' is the HDD.
Which OS are you asking about?
There usually some tips and tricks to tweak a system for better performance.

what you mean by slowest? what less influenciate or more influenciate?

well...
I am using windows 7 64.
I am actually on a Raid0 with 2x500gb seagate 7200.12 stripe 128k
i7 920 @ stock
3 x 2 gb OCZ gold edition 1333 but running at 1066 due to voltage!
I am thinking in buying a 3x 2 gb dominator or Gskill 1600mhz or 1866 mhz low latency ofc, and later (in 3 months or so) making a overlock with my i7.
The system is pretty fast I got nothing to complain about, but it can always be faster!
Do you think that after those upgrades my OS speed will increase a lot?

The two biggest things you can do to make your OS feel faster overall are to increase RAM quantity or increase hard drive speed. For systems with less than 4GB of RAM, I would usually recommend upgrading the RAM quantity first, and for systems with 4GB or more, I would recommend upgrading the hard drive. Your hard drive setup and RAM are quite fast already though - there's no real reason for you to upgrade at this point. If you really want to upgrade, using your existing drives as program and file storage drives and putting your OS on an SSD would probably be the biggest benefit though.

get a good ssd?

Hard drive will 99% of the time be your bottleneck. two western digital blacks in raid, a 10,000 rpm HD, or a SSD will prob give you the most noticeable difference in performance.

well If I get a SSD wich you recommend ?
and with those 2x500 in raid0 that I have here? I them just for back up?
is that a problem to mix ssd with those I got here?

Did you look at his system config? 2 WD blacks would not be faster than 2 7200.12s.

As for an SSD, you could still use your 500GBs as program drives. I would recommend an Intel X25-M.

but I would Just need one SSD? to run OS only? and could I let the 7200 on raid0?

You could do that, yes. I'll be setting up a tiered storage setup when I install Win7 next weekend, with an Intel X25-M as my OS drive, a pair of velociraptors in RAID-0 for programs and apps, and a couple of huge drives for music, movies, videos, pictures, and general documents (a WD Caviar Black 1TB at the moment, and a 2TB Barracuda XT as well when it ships - it's on backorder at the moment).

processor
hard drive
graphics
ram

definately the HD. Altho, without hyper thread, ram at all speeds is a numbered hoax(time for chore done reveals it, ignore bandwidth written)

influneciate is a strange wrod. it is just influence.  

multple threads to hd is most often raid, unless old school like me, you can twin two drives exactly the same on an one ide channel. master slave, or cable select (most likely today on latest ide). In this case, bandwidth written is low, but the chore gets done twice as fast. One of those marketed mysteries ya know?

heat is the biggest influence.

closing a system in for security narrowing amount of channels is also a huge perfomance sucker.

ramfile or pagefile, keep them off of the os hard drive, onto the root of another..this works on twinned drives, (would not recommend any other way)

dual channel memories, hyper threading, L2 cache for the older cpus at 1mb is always a winner,(single 512kb is done) ,multiple thread/core cpus..and hdds with ways I mentioned, and keeping channels open, not too paranoid in the securty stuff. that is all one can do to gain maximum. and defrag, like it was 1998