Often a stumbling block on the upgrade path is lack of backwards compatibility. If you are not willing to buy an entirely new machine, but choose to do it part by part, you still often have to change the motherboard, CPU, and RAM at the same time because of the sockets and interfaces used. Newer P4 boards, and soon newer Athlon boards, have DDR-2 memory interfaces. There are a few that have both DDR and DDR2, but they are being discontinued by Intel. Gigabyte is giving users a path to combat that. Their new PT880-Pro motherboard actually comes bundled with a converter that lets you plug in a DDR stick into a DDR-2 slot.
There is more behind this than just tricky wiring. DDR-2 has much different timings and voltage as compared to DDR. It is worthy to note that DDR2-667 pretty much out performs DDR400 to a degree, but you can get fairly fast DDR modules, PC4000 and up. If this board lets you clock DDR much higher than 200mhz, overclockers may have a dream come true in the GA-PT880Pro.