As I started reading this, I was thinking about how I'm in *exactly* the same situation, to the point that I actually checked to make sure I hadn't posted this myself. My brother bought me a mac mini when they came out as a gift, which rapidly caused me to relegate all of my linux machines to headless research machines. I bought a used 1.25 ghz 15" powerbook and have used it probably 6-8 hours a day since, when I'm not using my mini. The combination of those two machines have completely changed my computing life for the better. To make it weirder, I had *just* checked macrumors having just returned from the apple store to take a look at the macbooks, thinking about the exact same choices that you're talking about -- sell it now and get a macbook, or wait and sell it for less later.
All strange enough, but then I got to the end of your post. I don't play games. Period. Except for Diablo 2, which I just lost two hours to earlier today in Act V. Bizarre.
Since we're the same person, here's how I see it.
1 - the gpu on the macbook is probably fine for diablo II if you install boot camp.
2 - The macbook is nice, sure, but even at the higher screen res, I just don't see it as a great trade for the 15" powerbook, once you've gotten used to the screen size of the 15". Especially if you've got a high-res pb. It's not even that much lighter.
3 - Your PB won't drop *that* much in the next few months. If you're going to do anything, at least wait until the merom powerbooks are out, and either pick up a refurb for $2-300 cheaper, or just save your money and pay for it out of pocket.
That's what I'm going to do, anyhow.