I would advice sitting still for the moment. Most apps are not really structured to benefit from multi cores. As a result, you might not be trading up (2 cores to 4 cores), you might actually be trading down (slightly). Consider:
1/ There is a rough equivalence between a PPC G5 GHz and a Intel Core2Duo/Xeon GHz. Assuming the number of cores isn't much of a factor, since most apps aren't set to take advantage of it yet, you are actually REDUCING your CPU speed from 2.7 to 2.6!
2/ The PPC G5 is still more widely supported than the Intel core. For example, Photoshop and Microsoft Office still do not have Intel native versions yet. Hence both will run faster on your present PPC than on your proposed new Mac Pro. These are just two examples - there are tons more that are not yet Intel native (Universal = PPC + Intel Native)
3/ The new quad core Intel chips are out, and the rumor mill has it that an octal core (8 core) Mac Pro is imminent. Remember, the current Mac Pro is the first generation of Intel Mac Pro. It is due for a refresh soon.
4/ Finally, the Mac Pro you buy today will run Tiger, and you will want to upgrade it to Leopard when it comes out.
So, to summarize, I would wager that your present machine will outperform any new Mac Pro you can buy right now and will also run more software more quickly. Were you to buy a new quad core Mac Pro right now, you will kick yourself when in a few months a faster octal core is available for the same money AND it comes with Leopard pre-installed.
My advice? ... will be obvious by now of course ... sit still for another 6 months or so. Let Leopard come out and get its first few major bug fix updates released, let the next gen Mac Pro come out and go through any hardware shakeout that is needed, and THEN upgrade.
This is exactly what I am planning to do by the way!