I'm still doing some research as you I think.
I foud there are mainly two types of arm cpu:
with MMU, and without MMU. (memory management unit).
With a MMU cpu you are able tu set up OS such linux and winCE, and you get an envirorment very similar to the pc's one.
without MMU on cpu it is more similar to a microcontroller world. Sure you can put on it a lot of RTOS, and uCLinux too.
for development with the MMU ones for example you can get the embedded tools of mother (fk) microsoft in case of dev with winCE or the GNU toolchain in case of linux. (sharp has a PDA -zaurus- useing linux as OS and they give you the toolchain to develop on it)
for general development you can use @DS, IAR envirorment or the new metrowerks codewarrior too.
If your dsp is not too hard an high clock arm can be quite,
There are a lot of development system with and without mmu.
for mmu you can look at accelent (intel PXA) and cirruslogic (EP7312). there is still a cheaper borad useing ep7312 named "eagle" from another vendor.
For the mmu less I suggest Atmel and to take a look on links you can find at
www.ucdot.com
bye!
bye