The new DDR 2 4GB kits are on the way. The memory companies recognised the need for more memory as Vista eats quite a lot of it.
There will be two approaches. The first one is having a 2x2048 MB kits with DDR 667 or 800 timings while some of the more optimistic might bundle the 1000 modules as well.
OCZ for example already demonstrated 2x2048 MB while Corsair wants to enter this market with better timings. Corsair also plans to introduce 4x1GB kits with better timings.
Our advice is, if you go for 4 GB of memory you better go for Vista 64 bit as the 32 bit version won't see all of your memory.