How to tune Sitecore prefetch caches

What is the best approach to using sitecore prefetch caches? Given a stand alone Sitecore server with local SQL Server, 8gb ram and 2,000 content items, should I aim to prefetch the bulk of website content up front on the assumption that it is more efficient to bulk load items? For the sake of a clearer scenario assume that prefetching all 2000 items would take up 2gb as reported by /sitecore/admin/cache.aspx

Also, does the configured prefetch cache size in relation to required size also affect performance i.e. if set 3x larger than necessary, does this result in a performance hit compared to being set at 1.1x required size?