NAT issue?? help me out please!!!!!!

hi all,

we recently purchase a sonicwall firewall router(model TZ 170) here at work and i am responsible for setting it up. out networks at work are: 13 desktop computers hook to a 24port linksys switch. the switch is connected to a wireless/wired Dlink DI-524 router.

the router is further connected to our DSL connection. we also have four laptops and they get internet access from our access points. we have two dlink access points, one downstairs and one upstairs.

there is only two IT personnel here at this small business i am working for and i am one of them. i am responsible for maintaining the networks, troubleshooting computers if they go down. This is my first time working with a sonicwall firewall router and its comprehensive, but i really like working with it.

now, here at work our client computer IPs start with 68.xxx.xxx.xxx. we also have one server that downloads the emails from our mailserver in dallas texas and this server is set with a static of 68.xxx.xxx.17 ip. the server is using outlook as well as the clients to view and send out emails. i already configured some parts and entered our gateway IP, primary and secondary DNS ips.

also im configuring this router without hooking it up to the dsl modem because we want to hook the sonicwall firewall router up and everything should be good to go. our boss doesnt want the network down for too long. we are replacing the DLink DI-524 router with the sonicwall firewall router at work.

my question is with NAT. NAT and DHCP is enabled by default and instead of having the client IPs as starting out with 68.xxx.xxx.xxx, is it possible to set the client IPs as 192.168.1.100 etc.etc.etc?? will this interfere with the server we have at work, like our employees including myself not being able to read incoming mail or sending out mail?

thanks and hopefully ive explain everything clearly. if you guys need more info, let me know. johnwill help me out please. i read alot about networking but lack the extensive experience. this is one of the reasons why i got this job so i can alot by hand-ons experience. just like reading about driving, if you dont get behind the wheel.....................you know what i mean.

keymaker