Save your money and buy an aftermarket hard drive and install it yourself. Then you can buy a small external enclosure for the 320gb drive and have a nice external as well.
That's what I would do anyways, even if it did end up costing a bit more than $150.
5400 vs 7200 is only noticeable depending on what you are doing with your computer...
I'm actually considering that option... and aftermarket drive would be cheaper... however...would that void an applecare warranty? And how easy is installing ur own hard drive? I know its only like 1 cable connection but the concern is the space in the unibody and disassembling the unibody. Is it an easy diy?