The Q9450 is still a great CPU for games since it is what I have in my system. But at this point in time there is no real reason to build a new PC around a 4+ year old CPU.
I would opt for the i3-2100. The vast majority of games only use 2 cores. I bought the Q9450 so that I can play games and encode video at the same time.
Getting the i5-2400 as mentioned above is not a bad idea if you can save up money for it. There are a small number of games that can make use of 4 cores (compared to games using only 2 cores).