Although it is far from the cheapest, buying a Xilinx XUPv5 is a good idea if you want to learn a lot - not only is the 110T chip large enough to build very ambitious projects (a fully functional Nintendo console replica, for example), but it is also the board many top-tier universities use in their digital design class. Berkeley, for example, publishes their full notes, labs, etc for use with the XUPv5 board. I taught the class for a number of years, and would like to believe that it is an excellent, high-quality resource.
Hope this helps,
