the number to look for is on the low end. you really want your mains to reach down to about 50 Hz, even though you will probably set your crossover a littl ehigher than that.
You want to make sure there is an overlap between your sub and your mains, and 50 usually assures this.
the high end number might impress people to read it, but the sad truth is human hearing basically ends at 18 KHz, and for most people over the age of nine, it stops at 13 KHz.
another measure of quality is the sensitivity. you would like to see a minimum number of 89 dB, otherwise your receiver will be working very hard to pump out the volume you want.
the lower end of the bookshelf speaker market is probably the most hotly contested area in electronics right now.

You can actually get some really great deals in this arena from better names.
this one is particular will be hard to beat:
http://www.av123.com/products_product.php?section=speakers&product=1.1
here's another set for under a hundred:
http://www.bestbuy.com/site/olspage.jsp?skuId=5335693&type=product&id=1051806301325
you can also get some really great deals on the bic america speakers
http://www.thetwistergroup.com/product/DV-62SIB D00804.html
the old standby's like Infinity and Polk Audio can be good finds if you take the time to look for good prices.