since your input and output ranges overlap, you're going to have to use one of two technologies
flyback aka buck-boost
-or-
SEPIC (single-ended primary inductance converter)
check out the usual suspects, I've seen chips that handle the numbers you request ... Maxim, National, Linear, ON, etc
but, I don't think you nessecarly need a buck-boost circuit, your solar cell's open circuit voltage is going to be totally different than the under load voltage. if you have an array capable of outputing 5amps, then you just need a charge controller, which regulates the charging current to maintain optimal output from the cells... voltage takes a back seat to current in this situation (in terms of importance).