Need some guidance on solr-config settings

[Mark]

Can anyone offer some advice on what some good settings would be for an 
index or around 6 million documents totaling around 20-25gb? It seems 
like when our index gets to this size our CPU load spikes tremendously.

What would be some appropriate settings for ramBufferSize and 
mergeFactor? We currently have:

<mergeFactor>10</mergeFactor>
<ramBufferSizeMB>64</ramBufferSizeMB>

Same question on cache settings. We currently have:

<filterCache
      class="solr.FastLRUCache"
      size="16384"
      initialSize="16384"
      autowarmCount="16384"/>

<queryResultCache
      class="solr.LRUCache"
      size="16384"
      initialSize="16384"
      autowarmCount="16384"/>

<documentCache
      class="solr.LRUCache"
      size="16384"
      initialSize="16384"
      autowarmCount="16384"/>

<useColdSearcher>false</useColdSearcher>
<maxWarmingSearchers>2</maxWarmingSearchers>

Are there any other settings that I could tweak to affect performance?

Thanks