I'd get the 2.4ghz model. It doesn't seem to me that 5400rpm would slow you down much, and you can always upgrade the hard drive. But if sometime down the road you want a better video card, you won't be able to get it on the 2.2.
Get the 2.4 and upgrade the hard drive yourself when your budget allows. You can always upgrade the drive, but never the CPU or video card.