>    /topLevel/ents/ent[@name eq current()/@name]/value

Note that's an XPath2 expression. Use = not eq if you are using XSLT 1.

Also current() is an xslt-only function, you asked about xpath. If you
are using xslt you may be able to spead things up considerably by using
a key. 

<xsl:key name="ent" match="ent" use="@name"/>

then 
...<xsl:value-of select="key('ent',@name)"/>...

David