Is this an academic exercise, or are you intending to use the assembly language you learn? This is pretty specialized stuff, these days. Compiler optimization has become so good, that even something like a high performance hard disk drive would typically have around 1% of its code base written in assembler. Going way down the food chain to 4bit or some 8bit processors you'll see much more assembly language being used.
If you really want to have a go at assembly language, pick a processor that has a nice instruction set. I'm a former ARM employee so I'm probably biased, but I found that the ARM instruction set was particularly easy to learn - I was able to write an efficient FIR filter in asm on my third day at the company. ARM's semiconductor partners are shipping about a billion ARM cores per quarter these days, so it is pretty easy to find a company that is willing to use your ARM ASM expertise.
There are plenty of ARM eval boards available for around $50 to try your code out on - it looks like ARM Cortex M3 is going to be immensely popular. It uses ARM's Thumb2 instruction set, which is a little more obscure than plain ol' ARM32 code, but still pretty easy, and very powerful.
This little board looks very cool, for 80 euros: http://www.olimex.com/dev/stm32-103stk.html