For most games you should be fine, maybe a highly CPU bound game might bottleneck your CPU. But, since you are getting the i5 3570k, there's no need to worry. You can OC your q9400, till your upgrade.
Here's a comparison between q9400 & i5 2500Khttp://www.anandtech.com/bench/Product/76?vs=288
The 3570k would perform about 10-15% better than the 2500k.