The arduino pisses all over the first Z80 CPU I had in both speed and RAM. If your looking to get into microprocessors then maybe consider something using an ARM chip. Dare I say Raspberry Pi or one of it's ilk or maybe hold out for the Arduino ARM thingy (Not sure of it's name)
The Arduino ARM thingy is called the DUE, though I don't believe it is available for sale at the moment.
It depends on what you want to do. Do you want to do embedded programming like you've been doing with the Arduino? Are you wanting to run Linux or similar OS to run things like a LAMP stack (originally Linux, Apache, MySql, Perl/PHP, but more generally a web server)? What is your ultimate destination? Are you wanting to run Android instead of Linux, perhaps on a tablet? If you don't know where you are going, then it doesn't matter what you choose. :roll_eyes:
These days you can get evaluation boards for a lot of the embedded chips, though the development environments may or may not be free. I tend to think the ARM processors have the mind-share in terms of popularity, but there are other choices like MIPS and PowerPC.
For the embedded side of things, some of the ARM chips that I've bookmarked include:
Now I have the Rasberry PI, but I haven't had time to set it up yet. My impression is out of the box it is more aimed at the low end workstation than embedded programming, though as more people get their hands on the PI, this is changing. Adafruit has been adding pie-plates (equivalent to Arduino shields) and other things for the r-pi, and they have a tweaked Linux that they are experimenting with that is more setup to do hardware hacking than the official releases: http://learn.adafruit.com/adafruit-raspberry-pi-educational-linux-distro. I should mention that Adafruit also sells beaglebones and extras for that.