Whilst you can't upgrade to a Sandy Bridge processor there are still plenty of options for Westmere CPU upgrades that you can do to improve the performance of your machine.
I guess you may not wish to upgrade now, but so long as it's a 2010 machine you should be able to put in e.g. a Westmere Hexacore 3.33Ghz down the track. The model number is mentioned in multiple threads on the forum.