I haven’t worked on a quad-core machine yet, but the biggest difference I found when switching from a single to a dual core wasn’t so much the speed of one application when I’m running it hard, but instead the resources that are still available for other applications.
If I have Photoshop and Illustrator open, with a web browser, email, and who knows what else, Photoshop isn’t eating all the CPU cycles like it used to when editing big files, because it’s limited to one core.
I would imagine the quad core gives you the same flexibility but to a great extent. Memory is another matter and that might be an area where the $300 would be better spent if you’re not already maxing it out on your configuration. From doing the same types of work (web design and graphics work), a couple sticks of memory made a much larger difference in speed than a small bump in processing power.
Whether it’s worth $300 I guess would really just be up to you. I always look at how often I upgrade and how much that next setup is going to be.