Either you can select the nodeset like:
<xsl:apply-templates select="a[b='7']"/>

<xsl:template match="a">
..
</template>

<xsl:apply-templates select="a"/>

<xsl:template match="a[b='7']">
  <!-- This template will overrule the more general one, provided that
the predicate is true (that the child element b has a string value of
'7') -->
..
</xsl:template>

Cheers,
Geert