I have a Gaming PC and a 15" Powerbook.
Over the past few months I do less "work" on the PC and more on the laptop. Since the Powerbook can travel with me, it just makes more sense to have my email, calendar, my files and contacts all stored on it. My PC is pretty much used exclusively for gaming and voice chat (damn you TeamSpeak... release a Mac version!) now.
You say you do a lot of web and graphics work? Will the Mac be your new primary computer to do this work on? If it is, you definitely want a desktop machine. The Mini and the iBook are great little computers but they will not handle heavy load from programs like Photoshop (remember the old joke: "So, how much RAM does your system have?" "Almost enough to run Photoshop!"). If you're set on a laptop, get the more powerful Powerbooks. You'll also want a large visual workspace so go with at least the 15" monitor (and maybe a larger external monitor to do dual-screen with at home).
I love my Mac, it's great... but I recognize there are certain uses it's not suited for. Because mine is a laptop, it's not destined to be a high-end graphics workstation. It's a great workhorse for me that allows me to do most of my day-to-day tasks.