If you're doing heavy development, I'd recommend picking up the Atmel AT91SAM7X-EK devboard, solder a 512KB MCU onto it, and buying a nice compiler and JTAG unit. These can range in price from low-cost (GCC and one of the low-cost JTAGs) to really nice (the ARM RVDS compiler and RVICE JTAG...about $10k). We use the latter, but you can certainly do this sort of thing using GCC and Netduino on a budget if you don't mind using printfs for debugging...
Could you recommend a basic JTAG unit for single-step debugging that I could use with GCC? I want to get a feel for what this is like with a microcontroller, but have no idea where to start or where to purchase such a thing.