If the iMac supports triple channel, which pretty much every i5/i7 machine on the market does, then the best bet would be to get the 2x2GB now and then add 1x2GB pretty much straight away.
The Quad Mac Pro is looking horrendously overpriced right now. It was seriously overpriced before as was the rest of the Mac Pro line but now its just embarrassing. The Xeon's in the quad core line are only marginally more expensive than i7s and the only real benefit they have is ECC memory checking, which isn't of all that much use to most (even professional) people.
The 27" LED display is worth a fortune. If Apple were selling it separately it would probably cost at least $1200.
Even the Octo cores are now looking like an even worse buy than they were before. After all, consider this:
Octo Core 2.66GHz - $4699
2 x iMac 27" 2.66GHz i7 + 1 x Mac OS X Server 10.6 - $4497
With the second setup you'd get the benefit of dual displays using the input option on the second machine and if you hook them up with a gigabit ethernet cable then you could use OS X Server's XGrid component to use both machines for xgrid aware software.
Apple seriously has to do some major price cuts on the Mac Pro line. My guess is that they'll soon be dumping the quad core Mac Pros altogether and will have Octo Mac Pros starting at 2.66Ghz for $3299. Preferably even lower since $3299 is what the 2.66GHz Octo's should have been priced at originally to keep in line with previous profit margins and market competition.