Word processing and internet browsing? If that is the main use of your PC, I don't understand your line of thinking. All I would do is upgrade the RAM to 2 gig of inexpensive memory, buy more of the same you already have. That'll give you a nice little performance increase, beyond that, there is just no reason to spend more money, it won't do anything for you.
You also understand that if you do this, your OEM version of Windows is not going to work on the new motherboard? You will have to buy a new retail version of Windows.
Again, for what you do, just buy more memory. You are starting down a road that is going to get expensive, troublesome, and in the end, you won't notice a thing better than what you had in the first place.