[gmeroni]

It's probably not the ATV, unless you f*cked your unit somewhere.
FWIW a lot of wireless routers and ap's haven't got the needed processing power to deal with the complex WPA2 encryption algorithems and as a result drop dramatically in speed.
I have noticed this with even expensive units.
It wouldn't surprise me if your Belkin crashes under the heavy load of streaming WPA2 at max speed.

I'm thinking about to buy a new Modem/Router to let me stream all 720p & 1080p without troubles but i don't know what's is the best!

Anyone can advise me about a modem type? like maybe the Cisco Linksys WAG320N or Netgear ?

[yabbiman]

Since the ATV units don't support the N-standard I seriously doubt it would be possible to stream HD content over a G-standard (54Mb/s) WiFi connection. Maybe @dazex could give a clue to that?
Also, don't forget that encryption will drop effective throughput of your connection, even with the best hardware..

I'm always in to avoid WiFi whenever possible but you obviously have no other option as you pointed out.
Smallnetbuilder has a good router database which lets you input all of your needs since there's so much choice I'd like to point you their website
If money is no object take a look at Ruckus they make awesome wifi stuff.

[joshua.lyon]

Since the ATV units don't support the N-standard I seriously doubt it would be possible to stream HD content over a G-standard (54Mb/s) WiFi connection. Maybe @dazex could give a clue to that?
Also, don't forget that encryption will drop effective throughput of your connection, even with the best hardware..

I stream 720p content to the ATV2 in my bedroom over encrypted Wireless G with no problems.

I tried wireless N, but it didn't make any noticeable improvement on the 1080p front. On the other hand, I think that may be due to the cheap-o wireless N router I bought. The 1080p content works fine if I run a cable to the room, but we're mainly watching 720p content in there so it doesn't bother me.

Other people recommend using one of the Ethernet of Power adapter sets as they're usually a bit more consistent when it comes to throughput / connection quality. I haven't tried one yet, but MonoPrice has a pretty decent price on a pair of 200 Mbps adapters.

edit: I'm not sure what you meant by the ATV units don't support N. My ATV2 is connected to my N-only router and the ATV2 tech specs show that they support N.

[davilla]

Since the ATV units don't support the N-standard I seriously doubt it would be possible to stream HD content over a G-standard (54Mb/s) WiFi connection.

From the Apple Tech Specs on the AppleTV 2:

AirPort Extreme, Wi-Fi 802.11b, g, or n wireless network (wireless video streaming requires 802.11g or 802.11n) or 10/100BASE-T Ethernet network