There is a rule of thumb for Apple computer purchases that I have found to be very true. After researching the options, choose the computer that meets your needs, then buy the next version up. What you end up with is a computer that you can grow into and you will be happy with longer. If the 1.8 will suit your needs today, in a year, will it be long in the tooth when all the newer macs are running at 3 GHz? What happens while the OS is continually optimized to take advantage of the dual processor...Photoshop is already optimized for duals, and with the additional overhead being added to Illustrator, how long before that is optimized with dual processors in mind.
There's a certain mindset you have when using your Mac, that even though the 1.8 may be adequate right now and for a year, the newer models will always be better and faster. At least with the midrange model, you'll get more satisfaction with your purchase, longer. I've had a dual 2GHz since they came out and I plan to use it for at least another 3-4 years. My last Mac was a B/W G3 350 and I still have it running for postscript printing and my wife's use.