HI ALL
I am working in a small company and they do not want to spend much
money for firewall and all. now here thing starts for me i want to put alinux
firewall which can do contain level filtering .

we have taken mail services from usa based company who are not providing any kind of filtering coz of that we are getting hell lot spam .. and i heard there are gatway level firewall which can do contain level filtering but coz it costs too much we cannot go for
that ... so can u help me out to put firewall which can fullfil my all requirements

i have heard sth like snort .... which work s (don't know much)
now the thing is i have redhat 9.0 with default kernel what all the things. it's p4 with 512 mb ram ... now what i need
to make it good firewall....

Look at iptables for firewall. (Firewall doesn't really have anything to do with spam but rather attempts to hack your system.)

Look at SpamAssassin for preventing spam.