As was mentioned, there are some slot A T-Birds out there.
Look on the top edge of the cartridge at the model #.
A classic will start with K7 and the Birds start with A7.
The AMD Irongate chipset boards work well with the Birds.
A VIA KX133 chipset slot A board will only run a slot Bird up to around 800.
The exception is the Asus K7VT wich is modded by Asus for the slot Birds.
There is also a mod that can be done to the K7V to convert it to the "T" so it will run the slot Birds also.
Tom