Does that board have a 1/5 divider? This would force the pci/agp back to spec when the board FSB is set to 166. If so, that may be your best bet. If not, getting to XP2800 speeds may not be easy.
That board, if I'm not mistaken, will not unlock all the multipliers. But, since the XP2100 defaults at 13, then the board may read 13.5 for you just fine. If not, there are other options of forcing the board to a certain multiplier. Go
here for a guide that you can look over. First, change it to Socket View, Tbred, and change the FSB to 166. Lastly, where it says multiplier, change that to 13.5.
It will then display the sockets in your motherboard that you will need to connect in order to force the mobo to that multiplier. Use thin strands of copper cut very short (like from a strand of speaker wire). Bend them to U-shapes, and hope for a steady hand. Then, you plop the chip in, reseat the hsf, boot to BIOS, and set the multi to AUTO.
This worked perfectly for me. But I have a KT333 board and it has no problems with 166FSB.
You may have difficulty raising the FSB to 166, so try something in the middle if it's unstable. I recommend trying to hit 166FSB while keeping the multiplier low, like 10. That way, your chip is basically running the same speed, you're just testing to see if 166FSB is within reach (and won't overload your agp/pci).
To truly get some good overclocking out of these Tbreds, an Nforce2 board would really help you out. I've seen good ones for pretty cheap. You could sell that Soyo and defray some cost, ending up paying about $30-50 for a new mobo. Then, the only thing holding you back would be your ram.