Logic does tend to require a bit from your processor and RAM, not to mention it eats hard drive space like a mofo. I'm still working on my Mac mini, but I also have a MyBook 1TB external where I keep all my music.
I'd say get a 15" MBP circa maybe late 2008 with the 2.4 or 2.5 GHz processor, and because RAM is relatively cheap now, go for the whole 4GB (I believe it takes PC2-5300). It's going to cost you, but it will be well worth it and it should be a comfortable replacement for your iMac performance-wise.
Oh, btw, I use
this monitor with my Mac mini, and it's truly outstanding. Unless you're thinking about dropping $900 on the Apple LED display, it seems a good option for working at home.