Re: is there a big difference...
well, it kind of depends on what you do, if all you do is like go online, listen to music, watch videos, go online, you know the basic things, then you wont notice much difference than a few seconds faster loading, and starting windows.
If its like video encoding, then yeah, you will notice a difference, it will do it a few minutes faster (experience between my computer and my brothers)
If its gaming, then you will notice a big difference, though, the graphics card might slow you down a little.
Since your board has DDR, you are going to need to buy a new motherboard to run DDRII, and i pretty sure you are going to if you are getting that processor. Unless you were one of those that got a mobo that supports DDR and AM2 socket (not sure if they ever made one)
EDIT - huh... you posted before me.
Desktop -Cooler Master NV690/AMD Phenom II X4 965 BE/MSI 880G-E45 Motherboard/2 x 2GB DDR3 G.Skill Ripjaws X/Sapphire HD 5870 1GB GDDR5/CORSAIR CMPSU-550VX/Seagate 7200.12 500GB (OS), Samsung F3 1TB (Storage)/Sony DVD Burner/Windows 7 Professional 64-bit