Just to be clear - we are talking about a complete Vectra VEi 8 system rather than just the Sherwood-B motherboard.
With all the slots full (VGA + 2 x Voodoo + sound) your options are rather limited. I might have suggested one of those slot-mounted exhaust fans but you've no slots left. The PSU fan should be exhausting hot air from the case (and drawing it over the passive cpu heatsink in the process to keep it cool) but with a full set of expansion cards, these are all adding to internal air temps. I doubt you can increase airflow (and noise) from the PSU fan as this (and max expansion slot power consumption) is controlled by board circuitry - you'd either have to repalce the PSU fan or the whole PSU (wouldn't advise either option).
It's easy enough it fit a fan to the front of that passive heatsink with a couple of zip ties, though try not to disrupt the existing limited exhaust airflow too much in doing so.