Yes
If you have an ATX case, very easily.
Though you might consider upping the power supply to a min 400 watts. If you haven't already.
And, might I suggest NewEgg for online shopping if that's the way you're going.
NewEgg
Excellent prices, and great service.
Also, try to find the bleeding edge of computer goodies and take one step back. Buy that, it's a damn sight cheaper and all the irritating bugs have been fumigated.
As the others have said, you will have to format and reload Windows.