You can get mdf spacer rings for about $30 a pair allows you to mount a 6.5" speaker. To be honest I wouldnt want to go anything smaller than 6 to 6.5" component as your midbass will suffer (ie your music will sound tinny, hollow and empty). Treating the doors go a long way to helping the sound.
If your budget allows these Alpines are great
http://www.trademe.co.nz/motors/car-stereos/speakers/other/auction-833160858.htm
And if you decide to amp them later with 2x60rms (or higher) they will really shine.
Remember its not all to do with how loud, but also how clear.