I just updated to 511 and am not having any sort of memory issues what-so-ever.
I would double check to see what the amount of dynamic-ram preview memory you have set as Vegas will start to use that while going through its playback process.
If on Win7 go into Task Manager and then click on the Resource Manager and watch the tab for memory. See what happens there as it lists the amount used, amount allocated to processes, amount reserved, etc etc.
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http://www.davelozinski.com/DemoReel/
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