ZoneAlarm Pro Firewall eats up around 15-25MB of RAM (look for these processes in Task Manager-> "zlclient" & "vsmon").
Dunno about Avast, but AVG 7.5 takes only around 3-5MB (including all its process under "SYSTEM" user name).
With Avast there'll be very less change(neglegible) in start-up time, but ZoneAlarm slows the start-up by 5+ seconds.
If you have 512MB RAM or more, you wont see much difference in start-up time of your PC, provided there's no other Start-up process enabled in "msconfig". (except ZA & Avast
)