Memory speed shouldn't matter as much as in the raw processing of the chip.  While I love my X2 if I was buying a new box right now I'd look at a core 2 duo or the quad core, well its' 2 c2d cores glued together but thats another debate for another time, one depending on your budget.  

Here's a breakdown on DDR2 speeds to cost and what would be best in particular builds
http://forums.anandtech.com/messageview.aspx?catid=28&threadid=1966035&enterthread=y