The 8400 is slightly worse than the 7800GT which runs TF2 pretty well, so it should be fine. It's a PCI card though which means your computer probably blows (if it doesn't even have a PCI-E or, god forbid, AGP) -- your processor is probably the main bottleneck. When I upgraded to a 5870 I was still rocking a Pentium IV so stuff was weak. When I snapped in a new i7 920, it ran perfectly.