Popular choice for FW boaters and value for money on speakers are the Polk DB651 (NOT the DB651S). They're so good I put 4 in my car as well! Check the price on Amazon - often they come on sale for around $55 or so a pair.
http://www.crutchfield.com/p_107DB651/Polk-Audio-db651.html?search=polk+db651When comparing speakers, check out the frequency response in the specs. If you like a bit of bass, then you want something that will go down to 35hz or lower. They show in the spec sheet a frequency respons of 60 - 22k Hz for example for the Alpine speakers Ric recommends. IMHO, they will have a pretty poor bass response. Most of the cheaper 6.5" speakers seem to bottom out at around 60hz mark.
As a comparison, the Polk DB651 runs 35 - 22k Hz. The lower the bottom frequency, the better the bass will sound.
So my recommendation is to hunt around for a marine grade speaker that gets down to 35hz or lower.
Hope this helps...