If I have found the right motherboard then it will support a large number of processors (click here for a list).
The RAM issue - since you are running pro x64 I would expect you to be able to use all 4. Only 32 bit windows versions limit you to 3gb of ram. Make sure you are on 64 bit windows just to be sure.
I have heard that certain motherboards can have issues with RAM on win10 (see this forum post). I'm not sure what causes this, will look into it further.
As Warlord has said there is a risk that you will have issues with compatibility so it's up to you, if you do only need to upgrade your motherboard / cpu then it will be cheaper than a new system but if not then costs might rise. Your ram / gpu will almost definitely still work as long as you get a similar motherboard.