It'll hit the RAM and the Voodoo, sorta.
The Mach5 in your machine should be able to handle it decently, but yes, it does have an impact on speed.
I don't like Kal... it will cause crashes more regularly, you'll run out of RAM quicker, and yes, things will slow down in the thick of it.
Use an Appearance Manager compatible theme instead - they'll have little to no impact on anything.