[Skippeh]

Are you sure you want to do this? The .html files will be able to block search requests, for example a product.html blocks the searching of 'product' by this method.

If you still want to achieve this, replace your last rule with these two rules:

RewriteCond %{SCRIPT_FILENAME} !-f
RewriteRule ^(.*).html?$ index.php?SearchIndex=$1 [QSA,L]

i.e., only run the request through index.php if the example.html file (-f) does not exist (!).