Good plan of yours. I too am saving up. But it helps when I don't exactly know what it is I'm going to get with the money. It might be the iMac, it might be a newer MacBook Pro. Either way, it's like my "new mac fund".
And as someone mentioned above, it doesn't get easier when you get older. I'm pushing 40 and I'm still saving up for things. When I was younger it was much easier to save and spend I think.
That said, there was a time when I had nothing in my new mac fund. But now I have a good amount. Granted a lot of it was selling a lot of unused music gear I had, but the money was raised one way or another. My goal is to have enough for the quad iMac.
But just keep at it. I have no idea how long it'll take me to save the rest, but there was a time not long ago when I thought it would take me a lot longer to get where I am. As someone said above, it really does help to save and forget what you're saving for. Way easier said then done, but if you can just put away your money without constantly counting it and thinking about the end goal, and keeping busy with other things in your life, the time will go by a heck of a lot faster.