I am working towards my CCNP/CCIE and I just purchased two routers for myself on ebay. I acquired a 2501 and a 2504 as well as a MAU. The 2501 is a great basic router with two serial ports and an ethernet port. The 2504 has two serial ports, a token ring port, and an ISDN port. Between the two, I have several technologies that I can play with to prepare for my exams. I paid $1100 all together for the two routers and the MAU. I had to be patient and use snipe software to get good deals though, which I would also recommend to you.
One of the reasons I purchased the two 2500's is that they come with two serial ports. I plan on getting a third and I will be able to interconnect all three with back to back cables. If you plan on going further than CCNA that is something to consider as well as a switch(preferrably a 1900 or 2900 with enterprise). If you do decide to get 2500's, make sure you get routers with at least 8meg of flash and ram to be able to run images that support all the routing protocols. It would be preferable to get 16meg of flash for multiprotocol/enterprise images.
I am sure you could get a couple of 1600's that would do just as well, but I can't speak to them since I don't have any. I am happy with my 2500's though. Hope this helps.