>typically 50% of a windows PC(Mac 2Mb - PC 3-4Mb)
As Rojash says, it is Gb (Gigabytes) not Mb (Megabytes).
Before Vista came along I would say the average memory on a PC was 512Mb and 1Gb. It was rare to see a PC above 1Gb.
Then nearly 2 years ago Vista came out and people started saying that it needed 2Gb.
While some PCs came out with Vista and 1Gb (and still do) it is generally accepted that 2Gb allows Vista to run smoothly.
Most "general" PCs still offer 2Gb, it is mainly the expensive ones that offer more.
Some companies are offering 3Gb (as memory is so cheap) but I doubt most users would notice any difference between having 2Gb and 3Gb.
Offering 4Gb with the 32 bit version of Vista (that most people run) is a waste of time as Vista cannot recognise more than 4Gb of memory (and if you include the graphics memory a 4Gb PC will have over 4Gb).
For most Vista users 2Gb is fine, 3Gb comfortable, 4Gb a waste of time.
If a person runs the 64bit version of Vista then it CAN recognise more than 4Gb of memory.