Whether or not you can just change the chainrings or need a new crank sepends on whether the chainrings are rivetted to the crank (common on low end cranks) or bolted on.

The Trek specs page says this bike has an M361 crank. It seems this uses bolt-on rings, so you should just be able to replace the rings.

Go to this page:

http://techdocs.shimano.com/techdocs...=1215352626357

and click on the link to EV-FC-M361-2793. This will give you an exploded view of the M361 crankset and the part numbers for the available chainrings.

Be aware that buying chainrings individually can be expensive, and you should weigh up whether it would be more cost effective to replace the entire crankset. A complete Sugino XD600 26/36/48 crankset can be had for around $100. However, this will only give you a 2 tooth advantage over what you have now.

Try some test rides before you change anything. Your bike's 28 tooth granny is better than the 30 tooth that came on last year's 520, and if you are young and fit you might be OK.

As has been said, don't bother changing from 8 speed to 9 speed unless you have money to burn.

The front derailer question is simply one of whether the existing FD will play nice with the smaller chainrings. I would say try it and see. FDs aren't all that expensive anyway.