I would say, as many others have done, that the RAM sounds the best, but you should buy Leopard ASAP. Excellent chance that Leopard will absolutely blaze on an Intel Mac, and will be a lot more stable. Yes, OSX has been developed for Intel alongside IBM, but this is the first time that the development of an OSX system has been focussed towards Intel Macs, as opposed to towards IBM's chips.
Of course if you want to run Vista on your computer, the RAM is essential, and will make verything run faster in both Tiger and Leopard when you get it.
Anyway, Apple wouldn't leave a slot empty in the MBP if they didn't expect you to fill it real soon.