To me the upgrade to the 800 is definatly worth it, despite owning a 2000 I think the 800 is the "sweet spot" in the Rush line up (Cannondale want you to think the 1000 is).
However the Rush 600 / 800 question is as much a Sram / Shimano question.
I love my dual control shifters (I can't see myself racing with anything else), and the hydraulic brakes are amazing. However there are many users who are equally vocal about their disdain for dual control shifters.
If they have an 800 in stock take it for a ride, my wife has a bike with the full Shimano LX group and it works flawlesly.
The lefty on the 600 and 800 Rush is the same. However the rear shock on the 800 is the Float R. vs. the standard Float on the 600 (you likely knew this). The difference is that the 800 has a Float with adjustable rebound control (hence the 'R'). This will help you fine tune the ride.
Eitherway I am sure you will enjoy your near future rush!
NS2000X
PS: One final comment - don't buy the 800 just to have the bike sooner, in the grand scheme 3 weeks isn't that long. However in my opinion as stated above, the upgrade is well worth it, if it is compatible with your budget.