Thank you for your replies :)

Last night I switched to Firebird SS, I had tried some hard
operations and I could make about 240 connections with no problems.
But, today?! I'm having a hard time here. Sometimes the cpu usage
gets high, always about %45-65 and I when that happens , everything
gets real slow. When that happens, I kill the fbserver process and
when clients connect again, everything runs fine again for a while.

As an additional info, there's no swap space used yet. If I maxed
out, wouldn't it be used too? Let me show you my server info.

Cpu(s): 1.7% us, 0.2% sy, 0.0% ni, 98.0% id
Mem: 1035692k total, 1030072k used, 5620k free, 19340k buffers
Swap: 1048088k total, 0k used, 1048088k free, 901060k cached

PID USER VIRT RES SHR S %CPU %MEM COMMAND
9804 firebird 163m 59m 5648 S 0.0 5.9 /opt/firebird/bin/fbserver

Can it be something related with multiple CPUs?

Because of your memory concerns I've changed this setting :
DefaultDbCachePages = 512
(My database page size is 16384k)

Can I change this setting without restarting the fbserver process?
Oh, forgive me, too many questions for one mail, but how can I
start/stop the firebird service properly?

>> Well, let's see... 1GB (1000 MB) divided by 15 MB per FB client

yields at most 66 client processes. Assuming there is some other
stuff taking up a bit of memory, So either add more RAM or Swap
space. :)