DDR RAM is fairly inexpensive. Get as much as you can afford, it can be a critical element for performance. Forget the 133 stuff. Your system will be useful for a longer time if you load up with RAM. If your board only works with the PC1600 DDR RAM you can plug the PC2100 into it and it will scale down to PC1600 speed. PC2100 DDR RAM is going for $43 at Pricewatch right now.
Pricewatch
I have never used this board or even looked at one. Does it support DDR ram or is it a PC-133 only?
I am using dual channel RDRAM-800 in my systems.