The average memory volume of computer with Windows 7 is equal to 3.3 Gb, whereas for Windows XP this index is equal to 1.7 Gb, and for Windows vista - 2.7 Gb.
It is interesting that 86% of systems under Windows 7 regularly use 90-95% of accessible memory volume. Systems on base of Windows XP reach only in 40% of cases this level. This means that computers with Windows 7 operating system much more frequently experience the shortage of memory, and this lower their speed.
Certainly, the determination of optimum memory volume depends on the nature of work. We will not forget also, that for the access to the total volume more than 3 Gb is necessary the 64-bit operating system.