Core 2 E7400 vs Q9450

Generally, speaking you will only see an improvement if you play games or use programs that take advantage of at least 3 cores. Otherwise you could get slightly lower performance.

The Sims 3 uses either one or two cores, so having a quad core CPU will not improvement performance. The 200MHz lower clockspeed could potentially decrease performance, but Sims 3 is not a particularly demanding game. Like most games, the Sim 3 is likely to be more dependent on the graphic card than on the CPU as long as the CPU is not so slow that it bottlenecks the game and the graphic card.

On the little info you provided, you should see no change in performance. If you were to encode videos using a video codec capable of using more than 2 cores, then you will see a performance increase.

Also, I believe frame rates are capped at 30 for Sims 3. I could be wrong, but since it is not a fast action game, 30 FPS should be fine.

the q9450 will offer close to double the performance of an e7400 if all cores are utilised.

Well if hes willing to give it to you for free or a really low price then i guess its worth it.