its hard to say exactly what -it may be the other applications that are causing issues. You could seperate them out into seperate application pools which would allow you to determine which site(s) are causing the issues.

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you can use tools such as redgate ants memory profiler, however running a profiler adds an overhead itself. Realistically if you have multiple sites in the same application pool it is challenging to check - you can use windbg but it's not trivial. Its much, much easier to seperate them into different application pools and use that to determine the site(s) that are misbehaving - once you've identified the site(s) then you can use a tool such as ants to determine what causes the problem. As for system requirements, you simply need .net 3.5sp1 and sql 2005 or above - as with all .net sites the more memory (ram) and cpu the better - many people run on shared hosting accounts with low memory e.g. godaddy only provides 64MB of ram at their lower level (though that itself is very low IMHO)

However, the recommendation should be based on your sites needs e.g. a site with 1000 page views a day has different needs from one with millions (obviously) - and is variant based on what modules you use e.g. I debugged a customers site and traced all their performance problems to a custom module which was caching reports agressively - unfortunately as these reports were often 1-2MB in size the site came down at the end of each month when the sales staff ran their reports.

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