We wondered what happened with OCZ Vista Upgrade 4GB (2 x 2GB) kit. We played with it back in December. This 240-Pin DDR2 SDRAM DDR2 667 (PC2 5400) is available and selling at newegg for quite some time.
It is not even that expensive, it cost less than 4x1GB memory. You will have to spend $340 and a few bucks for shipping, and you will get 4 GB of memory in your machine. The timings are not great only 5-5-5-15 at 667 but with some overclocking you should be able to hit DDR2 800 speeds.
The memory is getting cheaper and it looks like the time of 4 GB memory kits has just arrived. OCZ also plans to offer a new one OCZ DDR2 PC2-6400 Vista Performance Gold 4GB Dual Channel with a better speed and DDR 2 800 timings but this one is yet to be available in the stores.
You can buy it here.