Shared hosting isn't equipped with XCache as standard, but you can set it up yourself in a jiffy. Several people have reported using the main caching systems with great success. I personally like XCache.
VPS for DB doesn't mean that the DB is stored on your own server. To be perfectly honest I haven't educated myself on how the service works or why anyone would even need it. Seems to me the first thing that would require dedication is the underlying script due to Apache processing, rather than the DB grabs.
Perhaps an admin could chip in on the benefits of that one
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How To Install PHP.INI / ionCube on DreamHost