I have also travelled during the chinese new year in the past, and managed to get tickets. The level of crowd depends on where you leave and the destination. Daniele is right, you should check the avaliabilities online.
Usually, the weeks before the chinese new year, it will be hard to get tickets from capital cities to smaller cities. So right about now it would be the worst time for those tickets 
After the 7 day holiday, the trend reverses, and it would be harder to get tickets from smaller cities back to the capital cities. In between large cities it is usually okay, for example, if you get the express train between Beijing and Shanghai, it would not be too bad. This is because the train does not stop at minor stations, and it is more expensive. Infact, it is possibly the case for all express trains. A few years ago I travelled from Beijing to Manzhouli during the chinese new year on an express, managed to get the tickets but just (got the tickets like a week before though).
Another tip that might help you: if you are at an inbetween station, you may "hop on" a train by buying a "no seat" ticket. You will be going on a "hard seat" carriage, it will be super crowded with no room to breath, but hey, it's better than nothing. You can also purchase a ticket to better seating after you get on the train this way. Some people also buy a "platform ticket" and get on the train "illegally", then buy the ticket when asked.
Hopefully that will help you plan your trip. all the best
[ Edit: Edited on Jan 18, 2009, at 6:39 PM by lzl ]