Sempron 140 is still only single core. It is much more efficient than a P4, so it would be able to keep up with a 9800GT. Problem is, you'll only be playing older games with that setup, as I said above, most games have dual core CPUs as a minimum. You could try unlocking the disabled core on your CPU if your motherboard supports core unlocking. Core unlocking isn't certain though, those core are often disabled for a reason, you have a 50/50 shot at a successful unlock at best.