Honestly? 4GB would more than likely be better for you, even if running 32 bit Windows. The 'speed boost' or whatever claim he made for the DDR3 isn't worth the cost you will pay. 4GB of good DDR2-800 or 1066 will work very, very well, and allow you to utilize as much as the OS can see. DDR3 isn't (in my opinion, which may or may not always be correct) worth the cost/performance delta over good DDR2 at much cheaper prices. Check Tom's; they recently did some testing on DDR3 vs. DDR2 and found almost negligible results on identical, or almost identical hardware configurations.
Of course, this only makes sense if your board supports DDR2 and DDR3 and can make that choice.