Yes, curently I am using a MacBook conected to the TV, but I can't get 1920x1080 resolution...I can select 1080p from the display sttings, but I get left and right black bars on the TV, so I set it to 1344x756 to get a full screen...That's because the MacBook has 1200x800 resolution and a 16x10 format, and the TV has a 16x9 format.
Again, no, that's not correct, it doesn't matter what the resolution of the Macbook built-in display is, mine is 1280x800 just like yours and I have no trouble driving a 1080p native HDTV display at 1920x1080
. Neither should you. You shouldn't need to set your Macbook to 1344x756, when you do that with a 1080p HDTV you're compromising the final quality of the image on the HDTV. If you care to try: plug your Macbook into power, connect to your HDTV and wait until you see the desktop on it, then close the lid putting it to sleep, wake the Macbook from sleep with the Apple remote (or a USB mouse plugged in) and see what happens, it should self-select 1080p, if it doesn't, then open the lid of your Macbook, its display will stay off, but you can use the trackpad to go up to the display menu icon drop down and select 1080p manually. Why you'd want to do this is to focus all your RAM and graphics capability on driving one display, the HDTV.
I doubt that a 21.5" iMac has the same price as a Mini+iPad 3+AppleTV.
I usually see the entry 21.5" iMac for $999 as a refurb from Apple, the low end 2011 Mini refurb is $519, the 16GB iPad 2 refurb is $319 and the the refurb aTV3 is $85. That adds up to $923. Put the extra toward RAM, a second hard drive, a mouse and KB, whatever. (Frankly, the iPad 2 is a steal at $319 and will stay relevant for a very long time. The retina display on the 3 is sweet, but we're talking bargain shopping here.)
My main consideration in suggesting this is to get an iPad in your hands, I find it invaluable.
after I realised that with mountain lion I can stream everything to the Apple TV, the iMac it's the better solution now, because I would use the iMac in another room, and stream to the TV from the livingroom via the Apple TV
The 2011 iMac is very nice, even now at 14 months old it's a good machine, and there's no doubt an Ivy Bridge refresh will be even nicer. We have a 24" iMac that's a few years old and still love it.
But, you'll still have to be able to control the iMac to tell it what to do, that isn't hard, you could screen share in with your Macbook, but it is something to consider: convenient, seamless control is worth striving for in the home theater, and the simpler that is, the better. We most often turn to the little Apple remotes to control our aTV3 and 2011 Mini running XBMC, and we always have one iPad in the living room to Airplay, act as a remote control and to screen share.