Flash refers to the controlling program running on the modem itself, which in most cases, can be updated. If your modem is connecting greater then 44K, then it may not be a good idea to change a good thing. 56K represents the maximum potential speed of a 56K connection. In the real world 56K is rarely seen due to FCC regulations. Actually, the speed given to you as the connection speed is probably not the actual speed you are running.
Modems negotiate their connections constantly, adjusting speed as necessary.
Modems increase the speed for good connections lowering the speed for poorer ones. Line static can even cause your modem to hang up the connection. Distance can also be a factor. The V.90 technology seems to do a better job of error correction. The overwhelming factor of any connection speed has always been phone line quality.