I5 3450 and iam talkin about cpu not gpu He was asking about GPU because that might be the bottleneck of your rig. Your CPU usage at 40-50% could be completely OK. So, what GPU?
SP or MP ? If MP than it's fine. In my case i5 2500k @ 4.5 GHz in SP CPU usage is up to 80% and 60% in MP that's because no AI and lower graphics in MP.