Presumably you have the installation discs for all your software and some way of backing up your data files? It's a bit of a gamble trying something like this unless you do!
This is an upgrade rather than a simple motherboard replacement? So in order to give some advice on where you might run into difficulties...
What was your old system like? How old?
What Sony model (in order to find out motherboard type)?
What processor?
How much RAM (how many modules)?
What type of hard drive?
What type of video card?
What type of sound Card?
What type of modem?
If these were all Sony standard features then a Sony model number might be sufficient to find out the details.
What new motherboard + processor did you get?