DDR speed backward compatibility

[vitec]

I'm planning to upgrade my 2.5 year sold P4 1.7 GHz, Intel 845 GBV, 256 MB RAM motherboard. But I need to do it in phases. The first and foremost I thought upgrading RAM would be a good idea. The current motherboard I have supports only DDR 266.
* My question is will this motherboard support a DDR 400 memory stick. (This new memory will be useful when I upgrade my CPU/mobo to the latest ones).

Also, I have a quation with dual channel mode in the latest mobos. Say for example there are four DIMM slots in the mobo.
* If I fill two macthing slots with DDR 400 and two other with DDR 266 - what will be the eventual memory speed ?
* Is it possible that the first two DIMM slots work at 400 Mhz and the other two 266 MHz or is it wishful thinking ?

thanks in advance

 

[Amaroque]

All DDR (1) is backwards compatible.

Your second question. All the RAM will run at the speed of the slowest RAM module. But, yes you can mix them.