Against 6 random hands, according to Poker Stove, 64s has 14% equity. Which indicates it's going to be a good limp on the button because the SB and BB are likely coming along.
Here's the specific scenario you posit (I think):
<font class="small">Code:</font><hr /><pre>
equity (%) win (%) tie (%)
Hand 1: 24.6148 % 24.58% 00.04% { 6s4s }
Hand 2: 24.7038 % 24.43% 00.28% { AcKd }
Hand 3: 32.0177 % 31.98% 00.04% { QdQh }
Hand 4: 04.3588 % 03.60% 00.76% { AhTd }
Hand 5: 14.3049 % 13.79% 00.52% { JhTh }
</pre><hr />
64s ain't that bad. This is a point Harrington makes in his tournament book that I think is somewhat applicable to cash games as well.
Regards,
T