A cheap way to check your chain is to get a ruler, and lay it with the zero mark centered on one of the pins in your chain. The 12" mark should also be centered on a pin if the chain is new, or in good condition. If the 12" mark is 1/8" or more past the center mark, then it's time for a new chain -- Now. If you keep an eye on your chain wear, you can avoid replacing cassette cogs and chainrings for some time. A worn, stretched chain will cause your cogs to wear unevenly, and if you leave the worn chain on there long enough for that to happen, a new chain won't fit well on your worn cogs. Sheldon Brown has some good examples (and better advice) on this at http://sheldonbrown.com/chains.html .

The Park chain checker is a good little gauge, but I tend to use the ruler method (my spoke length ruler does the job well) to keep an eye on things down there...

Tom