Haha, this might be the case for you, but actually it's quite hard to decide which parts to buy when you don't know much about computer.
And when you know a lot about computers, you realize there are 1000s of options out there to make 1,000,000s of possible combinations - so the buying is no easier. It is also no less disconcerting knowing that as soon as you get your brand new product home, the next version comes out and yours instantly becomes outdated.
I happen to prefer Gigabyte boards and Intels so we are good there. Using the eXtreme PSU Calculator Lite
, your PSU is plenty big for your current hardware. Even when I tweak the calculator with High-End board, 100% TDP, 100% (Peak) System load, and 30% Capacitor aging, I get 506W minimum and 556W recommended. Antec is one of my preferred PSU brands (and my favorite case maker) so your 550W from a reputable brand PSU maker is spot on. And it will leave you some wiggle room for two more sticks of RAM, and maybe a better (but not two!) graphics card. Your case comes with 2 x 140mm fans and 1 x 120mm, which is a great complement for great cooling.
What particularity/feature/stats should I be looking for when choosing a monitor ? I'm thinking about a 23''-24'' 1920*1080 resolution (to fully benefit of the GPU) but then what..? I'll be gladly taking suggestion/example of decent monitor.
Well, choosing monitors is even more of a personal choice. I would not worry about resolutions. Any current monitor will support your card. I like Samsung, but I like ViewSonic too. I use two 22" monitors and love that setup - I personally don't understand how anyone could live with just one monitor. But one 24" is not bad, once you get used to the limitations one monitor imposes on you.
Like the computer, you need to set a budget. A bigger budget will get you a better monitor. 16:9 is best if you will be watching DVDs and videos as that is the widescreen standard for TVs. 16:10, designed for computers, gives you a little more vertical space which some folks prefer, but also more black bars with video content, which a lot of folks don't like. I think gamers in general, are divided down the middle. So, like keyboards and mice, monitors are too personal - you need to visit a consumer electronics store and touch, feel and view for yourself.
I note however that your graphics card has two DVI and one mini-HDMI port. If you are absolutely certain you will NEVER go to a multi-monitor setup, then you can look for a monitor that supports HDMI. And note if it has built-in speakers, the audio will be carried through the HDMI cable, assuming that graphics card supports audio throughput. But if you might take the plunge and go for two or more monitors some time down the road, then I would look for a monitor that supports DVI. Mixing HDMI and DVI on the same card has been troublesome for many in dual-monitor setups. Maybe HDMI 1.4 will address that. HDMI came out of the home theater electronics world and the transition into the computer electronics world has not been as smooth as was hoped. Note the video signal in DVI and HDMI is exactly the same - same protocols and same quality - the only difference is HDMI also carries 5.1 audio, and HDMI control data. If a monitor supports DVI and internal speakers, it will also have a standard speaker input for the normal speaker cable from the sound card.
One thing I always look for in monitors, and it is hard to find, is height adjustment. Most monitors just tilt and swivel. I like to adjust the height of my monitors to my head (and that's after I adjust my chair height to my liking). I don't want to tilt my head up and down to accommodate the height of the monitors. If you wear bifocals, or worse, trifocals, that can be a real pain in the neck! No pun intended. If your desk has a hutch, height might matter there too. With me and two monitors, I also don't want a wide bezel, as some monitors have for "styling". I look for thin bezels so I have a smaller bar where the two monitors come together. Also, bezel width affects height too. So measure you desk space before you look at monitors.
Some monitors include speakers. This can be nice if limited on desk space - you can get rid of your computer speakers. But monitor speakers will never
sound good enough for "serious" music or video listening. They are great, however, for Windows sounds, Youtube, or casual background (though directly in your face!) music. That said, built-in speakers typically add significantly to either the bottom edge of the monitor, or both sides - depending on speaker placement.
Finally, I don't see an operating system. Note that only full "Retail" licenses of Windows can be transferred to new computers. If you don't have a full retail license, you will have to buy a new Windows license, in which case I recommend 64-bit Windows 7 Professional, or use one of the many free Linux alternatives.