It's hard to know if RAM will or will not get cheaper. DDR is pretty expensive compared to DDR2, but I'm guessing that they'll be cheaper later.
Also, Newegg has these:http://www.newegg.com/Product/Product.aspx?Item=N82E16820211188
which are probably a better deal (yes, 5-5-5-18)
Some motherboards don't overclock as well with 4 sticks of RAM as with 2 (something to do with the voltage supplied, IIRC).
I doubt you'd get much improvement with 3-4gb than with 2gb. That's the choice I'm going with. If I could only buy a cheap upgrade to xp64, I might consider it, but if that's an option, I'm missing it.
If you are trying to future-proof, be aware that Nehalem will only accept DDR3.