we have a max. of 98 octane-fuel at the pump here, how compression can my engine have with this fuel?

Octane ratings can be a little deceiving...RON vs...well, whatever the other one is. Anyway, the resistance to pinging is so dependant on many other things that the octane rating alone is a very small, but important, factor. Other than compression ratio, additional factors include: ignition timing, dynamic comp ratio, cam specs (which factor into DCR), induction precision, etc. I am building a 406 with aluminum heads, Holley 950 EFI and a large solid roller cam. With the right tuning and cam specs I will be able to run 11.2 compression on 91 octane. Not very different from the modern LS1.

Wrager is right. A general rule of thumb is about 10:1 for Aluminum heads and 9:1 for iron. The other factors can let you eek it up a bit higher depending on the application. If you search you'll find lots of discussions about the energy loss of iron vs. aluminum heads. Often, the HP gain of an extra point of compresson doesn't offset the heat loss through aluminum by much IMHO. But Aluminum heads are abundant and do combat some pinging to a degree. If you're getting ready to build a motor, spec the heads first, then move on to cam, pistons, etc.

-dnult