Soulrider gave you some good links to check out for further information.
A site I use to check prices is www.pricewatch.com
You can also get MB info at www.motherboards.org
Don't neglect the CPU mfgs web sites for links to "approved" MBs.
From the info at Soyo's web site, you are limited to a 700MHz thunderbird, or a 900MHz "classic" CPU.
Based on the price of Slot A CPU's, (they are not made anymore, and the 900MHz "classic" cpu sells for more than 2X the price of a socket A) there is no sence in trying to upgrade your CPU at this time.
Besides, there are other advantages to a new MB that will provide greater system speed increases than a fster CPU will provide.
If speed is your goal, I would limit my search to MBs that have ATA133 interfaces built in, and use DDR memory. These 2 items will impact system speed much more than the CPU speed.
I also agree with soulrider that you need to seriously consider a bigger power supply.
Depending on the overall system, you may want to consider a 400 watt supply. If you opt for a P4 system, you may need to change the PS anyhow to accomadate the extra power connector.
For modem information, check
www.modemhelp.org
www.k56.com
I have been happy with the Intel/Ambient HAM modem. Intel has drivers for W95-XP and Linux posted. It does not have caller ID though.
This modem sells for $13.25 at www.compgeeks.com