OEM (Original Equipment Manufacturer) versions of operating systems are intended to be bundled with new PCs, but you can also purchase them with qualifying hardware (hard disk, motherboard or barebones) and it enables a significant cost saving when purchasing an operating system. Be aware that the license agreement states that it can only be used on one system, and when that system is replaced, the operating system 'dies with the PC' and is not transferrable.

Upgrade editions are a 'retail' license and although they may only be installed on one system at a time, they can be transferred to a new system at a later date and also include licenses for both 32bit and 64bit versions of the operating system. (note that this requires the purchase of additional media in Vista Home Premium)

32bit versions of Vista are restricted to accessing a maximum of 4Gb physical memory - bear in mind that this includes all memory addresses in your system, so installing 4Gb RAM in a PC with a 512Mb graphics card would only show 3.5Gb available since the graphics card (and perhaps other devices with RAM) would use up some of the memory address space.

64bit versions support 128Gb RAM and perform slightly faster on processors with 64bit instruction sets, at the downside of having poorer compatability with some software (mostly specialised creative applications and games - day to day programs such as internet browsing and photo editing are usually ok) and driver support tends to be a bit more limited for the 64bit Vista, so we would recommend checking manufacturers sites for your hardware to ensure that this is supported.

We decided that Home Premium and Ultimate were the most relevant versions for our typical customer base. Both include media centre functionality and the Aero interface, but Ultimate adds additional security, domain login support and backup / encryption tools. See a full comparison chart here : http://www.microsoft.com/windows/products/windowsvista/editions/choose.mspx