You have to do a format when you switch out major components like a motherboard. There's no option to this.
If you don't have an ME OS Disk to install the OS on then no you can't just pop in the new motherboard with the old HD and expect it to work.
Also I don't think your 750MHz duron will work with that motherboard. Here is their CPU compatibility sheet for the NF7-S2 series which your board is apart of. Yours is -S2G because of the gigabit lan.
look at that it says:
and that's the lowest they support. The 750MHz you see on there is not a duron chip and is of a different core that's why a chip of the same speed is supported but your duron is not.
I believe a program like CPU-Z will be able to tell you exactly what kind of CPU you have meaning the core name of it.
Google up that name 'CPU-Z' and you'll find multiple places to download it