i dont know bout HW TnL but the radeon xpress 200 supports complete hardware-accelerated Microsoft® DirectX® 9.0 programmable vertex and pixel shader.
see the complete specs of the x300 chipset
here
and regarding the heating issues of the 3 ghz processors well it has been improved drastically from the earlier prescotts but the issue still exists. use a good ventialted cabinet with one 80mm front intake fan and one 120mm rear exhaust fan. that should be good enough.