connecting switch 3550 to firewall

Hello,
 I have 2 network connecting to centreal switch 3550
first network has switch (2950) at ip 160.16.209.254 mask 255.255.255.0  gateway 160.16.209.1
secound network switch (2950) at ip 160.16.211.254 mask 255.255.255.0  gateway 160.16..211.1

they are both connect to central switch  3550  ip 160.16.218.254 mask 255.255.255.0
switch is set to ip routing

port 1 of 3550 ip set to 160.16.209.1   (wich connect to network1)
port 2 of 3550 ip set to 160.16.211.1   (wich connect to network2)

Now I want to connect pix firewall 515 to 3550 to protect from a 3 network comming into outside interface of firewall.
(pix inside interface ip of 160.16.218.45 mask 255.255.255.0)
(pix ouside interface ip of 143.139.2.45  mask 255.255.255.192) (uncertain of this ...this is what was given to me)


I am uncertain of what gateway to give to central 3550 switch
in order to direct traffic wich is not destin for 160.16.209. and  160.16.211 to go to firewall

I guess i want to know if i should give gateway of central switch 3550  to ip of firewall and give gateway of inside interface of firewall to ip of central switch


Or do I creat a port on central switch  for example port 3 give it an ip of 160.16.218.1  and connect the firewall to it. give central switch gateway of 160.16.218.1  and also give gateway of firewall inside to 160.16.218.1

Or am I completly lost??
Should I be looking at creating VLAN instead? pros and cons to Vlan?
thanks