OCZ has just announced a new PC2-6400 Gold 4GB Quad kits where the company plans to offer 4x1024MB modules, all ready for Vista.
With Vista you might need more than 2 GB and definitely in this case you want to think about 64 bit version as the 32 bit won't be able to show or address all of your memory.
These 4GB kits will be available shortly but we don’t know the price at the press time but it should be a bit lower than buying 2x2GB kits so we expect it should sell for around €160 which is a great price for 4GB of golden memory. It will work at 5-5-5-18 with 2.1 V and the company claims it should be stable when overclocked.
You can find more about the memory here.