Can i have rams with different mhz?

Surely you can as long as they are from the same platform.. It doesn't even matter even if they are from different brands.. You need to keep in mind though that all the modules will operate at the speed of the lowest frequency module.. I.e. if you have two modules wherein one operates at 533 MHz and the other at 667 MHz then, the 667 MHz module will get downclocked (automatically of course) and operate at 533 MHz speed.. Also to check while mixing RAM's is that all the modules should be working at the same voltage levels and preferably (though not importantly) the same CAS timings..

what do you mean platform? ddr ddr2 and ddr3?

also how does dual channel work? i read somewhere that you need the exact same ram for it to work.
what if i have 2x ddr2 1gig 667 mhz ram and 2x ddr2 1gig 1066 ram for my 4 slots. will both the 1066 be downclocked to 667 even if they are in different channels?

Yup, unless you manage to OC the 667 up to 1066 speeds.

For the channels, it depends on which mobos you use, when mixing 2 sets of memory, you usually end up having the matching amount running in dual channel, while the rest runs in single. Though in your case, you would get a dual channel setup.

Or may be you could underclock the 1066MHz to 667MHz.. Both have their own headaches though.. One needs to match both timings and voltage for stable operation..

well i heard that if u put different mhz sticks then either there will be some freezes or restart probs or the mobo will run the whole setup on the lower clocked stick.....