I say buy now, Mac Pro is fine as it is now, and if Clovertown is the same socket at Woodcrest (Which it WILL) and you feel the need to upgrade then you can just drop a pair of clovertowns on your Mac Pro and sell the Woodcrests (which WILL work on a Windows PC).
It is not like Quad Xeons won't be great for a long long time, not to mention there will be barely any apps that will take advantage of EIGHT cores and when there are you can upgrade.
See thats the great thing about Mac Pro and what eventually led me to get it instead of an iMac.