so, for a solo miner using bitcoin-qt, that decide what goes into a block? is it configurable? 
It's not. You need to modify the client from the source.That's incorrect. The following command-line or bitcoin.conf configuration parameters let miners set their transaction acceptance policy:
Quote
-blockminsize=<n> Set minimum block size in bytes (default: 0)
-blockmaxsize=<n> Set maximum block size in bytes (default: 250000)
-blockprioritysize=<n> Set maximum size of high-priority/low-fee transactions in bytes (default: 27000)
-blockmaxsize=<n> Set maximum block size in bytes (default: 250000)
-blockprioritysize=<n> Set maximum size of high-priority/low-fee transactions in bytes (default: 27000)
The -mintxfee / -minrelaytxfee options currently control how large a fee-per-kilobyte is needed for a transaction to be considered "fee paying" versus "free".