Get it now.
I don't think the Prescott should be a factor in deciding on an Intel system today. Remember that only the first generation of Prescott CPU's will be built on the Socket 478 platform. These chips will be built on the 0.09u die and ship with higher default speeds. It's uncertain if these early Prescott's will offer the 1MB of L2 cache or a higher default FSB speed.
The biggest reason to wait for the second-generation Prescott will be the features on its board. DDR-II by itself will be worth upgrading for. PCI-X and SATA-2 are won't offer an immediate increase in performance, but by the end of 2004 both will be firmly established as standards.
Unless you buy a new processor and motherboard every six months, your best bet is to buy a P4-C and 865/875 board today and move to the second-generation Prescott and new board late next spring or early summer.
BHD