Ok.....there is two types of rear wheel drive inlet setup and 3 types of front wheel drive setup.
The basic engine underneath is the same beast (differing bore and stroke dependent on cc)
The original C25XE as fitted in the Cavalier and calibra used the red top type inlet as per:

There is one expcetion to this and that is th Calibra which also used this inlet setup on an X25XE from approx 1995 until last porduction around 97/98.
Then there is the X25XE inlet as used on front wheel drive applications (Vectra in this case) identified by the use of an accelerator cable.

The Y26SE in the Vectra used a setup which looks the same but, has a throttle motor to support the drive by wire throttle as per:

There is a further varient used in the front wheel drive for the 3.2 V6 (Vectra C and Signum) which looks totaly different again.
The Omega ones are easier.....they all look similar!
The 2.5 and 3.0 shared the same inlet config until mid 1997 when the 3.0 adopted a longer front to the plenum as per teh photo below:

The 2.5 kept the earlier shorter plenum setup.
The 2.6 and 3.2 Omega setups also used the longer plenum as per above but with the addition of a throttle motor to replace the throttle cable.
There are some others to note to, i.e. the V6's in the Saabs and Sintras.....these have variants on the Omega setups normaly