Top n values

I need to process the top n values in a list where n
is dynamic. So if n=3 and I had the following XML:

<root>
 <item>1</item>
 <item>5</item>
 <item>3</item>
 <item>2</item>
 <item>6</item>
 <item>4</item>
</root>

I need to process <item>'s with values 6,5,4. If I had
captured n in a variable/parameter, what is the most 
efficient way to do this? What I want to avoid is walking 
the <item> nodset multiple times as there are a lot of
<item>s (thousands). Thanks.

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