> We have an firebird DB running 2.1.2 on a Linux server.
> Its soem 25 Gb in size.
> I has on average 100 - 140 sim. connections.
>
> At the moment we use SS.
> But we are having some performance problems, which I would like to hear if they could be prevented if we switch to classic server.

Without description of the problems it is impossible. May be some
tuning of FB/Linux/hardware configuration would be more effective.

> Sometimes the CPU is at 100%. And if I remember correctly, the SS isn't able to use more than 1 CPUs whereas classic will be able to use both.

This is true for Windows, but I'm not sure about Linux. It is quite
possible that Linux kernel can spread threads on different CPUs.

> And if we switch should I have something in mind regarding the setup (i.e. assigned memomy etc).

Classic have separate data cache for each connection which mean that
if you configured big cache at DB level and don't change it, you can run
out of RAM.

SY, SD.