Difference between PIX and ASA

Hi,
can someone explain me the differnces between a PIX and an ASA, especial
a PIX 515E/R and an ASA 5510 plus.
I understand, that the ASA has the full functionality of a PIX plus
SSL-Gateway and the option of installing SSMs.
I have the problem to combine and expand our PIX based network with a
watchguard and soho-router based network. In the first step i will
replace the 2port Watchguard on ..
Telecom, Cabling and VoIP continues...