P3 800MHz, 384MB SDRAM
I tested with 2 graphic cards:
1) ATI Rage PRO 128 -> 13 fps max in game (30fps in tactical view)
2) GeForce2 MX -> 50-60 fps in game (~100fps in tactical view)
in the later case, map was rendered in low definition, but particles and sky were on whereas everything was set to low with the ATI card. Also I ran those tests on an Kubuntu Edgy box.
It seems that what matters most here is accelerated T&L (an intel GMA900 is slow too because of software T&L) since TA3D renders lots of geometry details (try typing wireframe in console ...) but with accelerated T&L it's just fine
Also a P3 800MHz can handle multiplayer games at normal speed with quite a large number of units
We should write minimum system reqirements somewhere in the docs