The quickest and cheapest way to an almost spot on Hammond sound is almost definately the Native Instruments B4 software package...we can hark on forever about The Tyros this and the Tyros that...the Tyros is a great keyboard with a big price to match, making Hammond sounds is just one of its many aspects so it just gives you the equivilent of just the upper manual on a Hammond to compliment the other sounds whereas the NI B4 is totally dedicated to providing the 2 manuals and a pedal board as per a proper Hammond console which only a seasoned Hammond player would appreciate...if the the occasional Hammond sound is all thats required in your music then most modern keyboards will provide a passable imitation, if you want close to the real thing go for the B4...you can download the demo and read user reports here....
http://www.kvr-vst.com/get.php?mode=show&id=51

Rgds
larigot