You won't notice a difference and the hard drive upgrade is not worth it.
If you know a bit about computers, you can buy yourself a hard drive (320GB for $45, 500GB for $55) and install it yourself. So for $45, you get a 320GB HDD and you get to keep the hard drive that comes with your computer (250GB). As you can see, Apple's upgrade is a rip off.
You can find the hard drives on Newegg.
If money is not an issue, consider 8GB of RAM and an SSD.