Though I would never recommend a laptop for gaming I would say that since most laptops such as mine with a 15.4" screen have a relatively low res that the CPU will be very important. The one I am currently on has 1.5gb 667mhz DDR2 memory 5-5-5-15 timings, 120gb SATA 5400rpm 8mb cache hard drive w/ winXP Pro, an Intel Core 2 Duo T5600 1.86ghz Dual Core with 2mb L2 Cache and an ATI X1600 Mobility. I have had the laptop for slightly over a year now and it runs most games decently. My native res on this beast (lol) is 1280x800 and that is going to be most of the laptops this size native res. If you can afford the higher CPU it will help, but you should make sure to have atleast 2gb of memory, and a 6 series GPU or higher (i.e. x1600, 2600, 6600, 7600, or higher of course) anything below will not allow for a very good gaming experience even if you dont play very often.
So to answer your question I (personally) would spring for the 2.4ghz if I had the cash, but if you are hurtin for cash (since it is the holidays this is understandable) then the 2.2ghz would work fine, but even though the performance difference would be little, it would help since the overall performance wont be to high as it is. I had the same decision to make and I think overall it helped. I could have got with a 1.6ghz dual core at the time I believe it was a Core Duo and then I had the chance for my 1.86ghz processor. It took alil more out of my pocket that I actually had, but it wasnt to hard to make up for and it has paid off I believe.
I know this is scrambled, but I hope it helps.
Best,
3Ball