I'm no expert on these things, but Tom's hardware recently ran an article in which is said that performance increases form ddr3 weren't yet worth the cost.
My guess: It's not really clear what the world is going to look like in 12-18 months, since Intel's new Nehalem architecture is due out this year. This may or may not influence RAM requirements and prices. I know that Nehalem will move the memory controller from mobo to die, but I don't understand this stuff well enough to know what that means for RAM; it's supposed to re-dig a grave for AMD and bury them six feet *further* under with seven extra nails in the coffin for good measure (but never believe the hype.) And of course, I have no idea whether Nehalem is going to have reasonably priced/performance processors or whether a Wolfdale is going to be the proc for the near future.
On the other hand, RAM is SOOOOO cheap right now, you might as well just grab 2x1GB CAS 5 DDR2 800MHz for $50.
So there's the opinion of a guy who prefaced all of this with "I have no special clue what I'm talking about.