But do you also know if the result will be correct if I try a 1280*1024 resolution when playing in surround mode on my 22" and use the native resolution on the 22" (no triple screen) for the surf, etc...
Yes, that will work fine. If you have dual-input monitors, you can even hook up one of your side monitors to your secondary GPU output and run a dual-monitor desktop with each screen at its native resolution.
With an inexpensive secondary GPU you can drive your third screen this way as well. Telling windows you have three real monitors instead of one ultra-wide one also has big benefits pertaining to 2D desktop management software (because Powerdesk sucks the big one in this area).
Is my 8800GTX 320Mo sufficient to play in 3840*1024 resolution
Yes, although you probably won't be able to max settings on the newest titles. But you should be able to get everything playable with reasonable settings.