You CAN downgrade from DirectX 8 to DirectX7, I've never done it but I know that it is possible. There are utilities out there that basicaly 'remove' directx 8 and then you just install directx 7.
Here are the drivers for your card:
http://www.sis.com.tw/support/download/530.htm
They officially support DirectX 7, but it WILL still work with DirectX 8 (or at least... it should, no reason why it shouldn't)