TweakTown has some low-down on the whys of 4GB memory setups and how you can get beyond the 4GB limit, which is not hard with today's technology:

As for the transition to 4GB, it is definitely coming. Windows Vista is a memory and processor hungry beast, 2GB is the absolute minimum you are going to want to use with this OS. Forget Microsoft's recommended 1GB; that is just to get it installed so you do not get those pesky error messages saying you do not have enough memory, 2GB is needed to get anything done, and 4GB is going to be the real sweet spot. 32-bit OS users have two options, live with only having 3.5GB of memory accessible or upgrade to 64-bit.

Microsoft's strategy on this really leaves us in the dark. Vista needs 2GB for good performance and 4GB for optimal performance, yet its 32-bit version cannot access it all. Only the 64-bit version can do that, yet you cannot get this at a retail outlet, only the 32-bit version is available, which includes an application for 64-bit discs. Only the Ultimate Editions of Vista come with the 64-bit in the box. Also there are no advertisements for 64-bit by campaign or by website, so why is Microsoft delaying the adoption of 64-bit if its own OS needs it? Sure, right now there are not a lot of fully optimised 64-bit programs or games out there but there is some and until Microsoft really start pushing it, it will take longer for other companies to really start supporting 64-bit computing also with more finely tuned 64-bit version drivers and applications and so on.

Should you buy a 4GB memory kit like what Mushkin are offering? If you are going to run Vista, look at holding off till you get a 64-bit version - it is not worth installing the RAM and not having access to the full amount under the 32-bit and then having to upgrade or format later - do it all in one hit.