The general rule of thumb is to add indexers before you add search heads, because additional indexers allow searches to complete faster via distributed search. Faster searches mean that you have less concurrent searches on your search head, reducing your need for additional search heads.
I'd recommend option 4:
server 1: indexer, server 2: indexer + search head
As someone else already mentioned, you should also consider adding cores and RAM to improve performance. See: http://docs.splunk.com/Documentation/Splunk/6.2.2/Capacity/Referencehardware
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