For a super 7 motherboard and a K6-3, I would suggest a Voodoo 3 3000. Should be able to find a brand new retail version for around the $100 mark, and an oem for around $70 or so. Used retail ones also go for around $60. The Voodoo 3 2000 can also be had for around $70 retail, and is only a little slower (143 MHz vs. 166 Mhz) and used ones are around $50 or less.
Chuck
PS, I do happen to have one for sale, PM or email me if interested