Congrats on your new purchase! Excellent machine. Just like Lord Nerdos I too waited from Autumn 2006 and eventually gave up in April. I'm well chuffed with my Mac Pro 2.66 Quad with 3GB Ram (bought extra from Crucial) and added extra HD. With the cost of HD's falling so fast that empty 4th bay is looking good for a 750GB or even 1TB time machine drive.
I'm still over the moon with my machine and you will be too. Boy is it FAST! Don't worry about updates, as has been quite rightly pointed out any increase in performance isn't going to make THAT much difference to the current line up.
Who knows - with Leopard, performance of the OS might even improve (if that's possible) with better multi-core support, 64-bit and the like.
I'm just annoyed I have to use Windows here at work - going back home and firing up my Pro is a great way to de-stress 😉