Hi all,
I'm trying to improve silence of my desktop so I need your help.

This is my configuration :
PSU : LC Power Arkangel 850w
CPU : Intel Q9450 2.66 Ghz cooled by Arctic Freezer Xtreme v2
Motherboard : Asus P5Q-Deluxe
GPU : ATi Radeon HD4870 Sapphire 512mb cooled by Arctic Cooling Accelero V2 +Schyte S-Flex 120mm 800rpm
RAM memory: 2x2gb(4gb) G.Skill PC2-8800 1100MhZ
Hard Disk(s) : 500gb Seagate 7200.12 16mb cache + WD 2TB Green Power 5900rpm
Cabinet : Fractal Design R2 Pearl Black with 2 stock fans on front and back
DVD combo : Pioneer 215-DVK Black SATA

Which improvement can I do for a better silence computer? Maybe I can change the seagate hd with a SSD OCZ Vertex 2, but what else? I want less noise possible.
Thanks to all for your help and sorry for my bad english.

Changing out that 7200rpm drive will certainly help, and even an inexpensive SSD model will also be a good performance boost. Other than that it's a question about what noises you are hearing right now. If case fans are running at full speed you might want to take them down a bit. The fan on the Accelero graphics cooler probably doesn't need to run at full speed either, if you could turn it down just a little. I hope that oversized powersupply stays quiet most of the time.

How do you control your fans?

I connect case fans to the motherboard and set them to standard profile via BIOS.
CPU Fan is set to Silent profile via BIOS too.

Ok: generally speaking your current setup seems to me well balanced (maybe the 4870 is outdated and underperforming nowadays for its thermal envelope: you may think whether or not swapping it for something better from a performance/efficiency standpoint, and you may easily reuse your very good GPU cooler) with some remarks.

EDIT: First of all, as already noted, your PSU isn't a silence-oriented one. You do need a quiet one in the 200-300W range.

It would be a worthy investment if you could resell your current PSU, IMO.

Probably I would also use a less aggressive profile for the case fans too, and/or use a software like SpeedFan to have a somehow greater flexibility.
At anyway, IMO more probably that not you don't need the intake fan, providing that your 4870 already has its one.
Disconnecting the intake fan you might observe a marginal temp increase, but nothing serious (providing that GPU's VRM are adequately cooled by specific heatsinks).
Eliminating the intake fan you could get a noticeable improvement, as it's on the more straight path to your ears.
On the other hand the Fractal stock fans are already very quiet, so you might also not observe such an effect.
You have just to try whether or not it may be effective.

Even about the 7200, as already said, you can get rid of it, swapping for an SSD (or two small one, like some Intel 40GB, in RAID 0) and gain substantial improvement about actual noise profile (not to mention the increased performance).

Along with it, maybe you could/should swap your AC Freezer Extreme: it's a rather inefficient design, and moreover and it's a bit noisier than necessary at any speed above 800rpm. Otherwise, according to SPCR, its tonal quality is excellent.
Personally I would change it, but I think it's maybe a relatively new purchase, isn't it?
Here you can see SPCR findings about it (take note of the original fan levels): http://www.silentpcreview.com/article1128-page6.html
Here you can see how it compares against other coolers (from a silence perspective): http://www.silentpcreview.com/article1128-page7.html

IMHO it's not on par with your other components (always from a silence perspective).
Just FYI, I've recently bought a Cooler Master Hyper 212+ from ePrice at 24 euros, it may worth a look (but as for the AC itself, even that CM deserves a new fan, IMO, to fully exploit its quietness potential). Or a Scythe Mugen 2 Rev. B, I've found it as low as 40 euros.

Guys, I've a doubt..
In those years I change my way to use computer, expecially to gaming.
I'm not a gamer anymore, I use my PC for watching movies/tv shows, listen music and using office software for college.
Maybe it's time to sell everything and change to a mini-ITX configuration?
I need silence and not too bad power consumption.
It's better spend my money to improve my actual PC or it's better trying something new which fits to my new requirements and sell all my stuffs?
Thanks again.

It's difficult to say, IMVHO: first of all miniITX are maybe more difficult to cool quietly due to dimensions constraints, but nonetheless miniITX is gaining more and more momentum even among silencers.

On the other hand, the power consumption might be very low (I'm thinking of newest Atom and Zacate, not of more powerful platforms), so the heat is much lower (and the same goes for the needed cooling power).
Intel Sandy Bridge is very low on power consumption also, but it costs much more.

Unless dimensions/placement is a key indicator/factor, personally I still rather standard desktops, with not too big enclosures, and a careful parts picking, for my silent computing.

I think your current setup was probably good for its time. I think a current Sandy Bridge might end up also being a well timed investment. The next generation Ivy Bridge is going to be pin for pin, board for board compatible... only about 20% faster. A Sandy Bridge should age gracefully as it takes you through college.

I personally like ITX. Smaller, more portable, easier to find a place to put it. It probably ends up costing a little more than a micro-ATX, but in my opinion the convenience is worth it.

I would take a look at a new Sandybridge system

You need to start with what is making the most noise. Swapping out the Seagate for an SSD should be an easy decision.

How about your PSU? How about your Arctic Freezer Xtreme v2?
How about your stock fans? Maybe you can replace your stock fans with two 800rpm slipstreams?

Hi Ces, thanks for your advice.
For my PSU I dunno which brand should I buy. Maybe a Corsair CX500 is good?
Same thing CPU cooler. which one is better for sound level ?
And sure, I will replace both stock fans with 2 slipstreams.