Don't play the Waiting Game and get one when you need it. There probably won't be an 8 core MP released anytime this year. If you must, you can buy the two quad-core CPUs and drop them in later (according to initial reports, this will be possible).
1. Apple tends ot to use new Intel chips the second they are released (Mac Pro was announced about a month or so after Woodcrest was introduced, and the MacBook and MacBook Pro have yet to receive a Core 2 Duo upgrade - perhaps next Tuesday). It could be some months before Apple use quad cores.
2. There is no guarentee that Apple would put 2 Quad core CPUs into the Mac Pro. They could produce a single-socket second generation system (as they did with the dual core G5 Power Macs).
3. The Quad cores are not going to clock as fast as the current Woodcrest CPUs. They may not thus fit into Apple's plans for the Mac Pro - though servers are another question.
4. Given their track record, Apple is going to hold off any major upgrade to the Mac Pro (and the iMac) until after Leopard is launched (got to get all those people holding off buying until Leopard/CS3 is released). Then they hit you with a major update a few months later.
I would be surprised to see an 8 core Mac Pro within the next 9-12 months.