We have servers on Win 2008 Std which has only about 3-4GB RAM and it runs perfectly well. Nothing of a problem. 1 of them has about 200+ sites on there and it's still doing good with only 4GB RAM. Not too sure what happened to yours but I am suspecting that 1 of them much be really really using or doing lots of processing and etc.
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Windows 2008 does RAM caching so you need to check the 'actual' usage.
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Not to mention that if you're running .net apps through IIS, you have to contend with the CLR's GC and how that works. For the most part, late generation objects will just sit there until the next GC flush.
There's so many performance counters you can plug in just to watch how it unfolds. What you want to look at is the 'Private Bytes' to make sure it doesn't continually increase over prolonged periods.