The Voodoo3 and the GF256 are extremely power (as in electricity) hungry graphics cards. The GF256 more so than the Voodoo3. It takes the nearly maximum power allowed in the agp spec. the thing is many super7 boards cannot supply the maximum power specified by the agp standard and thus cannot supply enough for the card. It is ok for low load applications, like normal windows tasks and normal office apps etc. But, when the card is being heavily used, like in games, the system hangs. Sometimes it is the powersuply unit. It cannot supply enough power to the board.
Anyway, for starters, get the latest version of the VIA drivers (4in1). Get the latest version of the video card drivers. Check if everything works. If not, check if you have any IRQ conflicts. Try to ensure the graphics card is not sharing its IRQ with anything else.
If still not working, take off the extra fan you put in above the video card (the card should have one onboard?), and move all the pci cards as far away from the voodoo cards as possible, i.e. put them in the slots starting from the left end. now that should allow more air to go through the cards sink. Some super7 motherboards had a jumper for voodoo3 mode, check if yours has that, if so set it. Check if everything works.
If still no help, check if the PSU is not powerful enough (~200W-250W or less). With those cards you'll probably need a good 300w power supply. Before you take this advice, do you know if you have an AT motherboard or an ATX? If its AT, its probably gonna cost you too much getting temporary upgrades which you probably will have to disgard pretty soon. Check if cards with a lower power-apatite like the GeForce2mx, ATI Radeon, Kyro2 or Kyro will work on your system. If not, you'll probably need to get a TNT or something.
If the system is ATX, then it would be worth while investing in a good 300W powersupply unit like one from enermax. They make good reliable PSUs. Even if it doesn't come in handy for this system, it will serve you well in future ones.
If you're still having problems, I'm afraid you'll probably just have to get a card that isn't so power hungry, like one of the ones I have listed. Don't worry though, all the cards I have listed are better than the Voodoo3. Unless ofcourse you want to go for a total system upgrade.
<font color=red><i>I refugee from Guatanamo Bay,
dance around the border like I'm Cassius Clay
</i></font color=red>