How much memory does pre-installed software take up?

Are you referring to RAM which is commonly known as memory or are you referring to hard disk space? I suspect you are referring to disk space. However to answer your question I believe it's between 10-20 GB is taken up by OS X and iLife which is all that's loaded on a typical Mac on delivery. What will take up the most amount of space on your new MBP (or any computer for that matter) is media such as music, movies and photographs. Many people load their media libraries on external HD's which frees up the internal HD dramatically, if you have a large or growing media library I'd urge you to do the same. External HD's are inexpensive these days and if you use Time Machine to back up your system it will also take care of backing up the EHD too.

Roger

Disk Inventory X counts disk space differently from Snow Leopard. Until the introduction of Snow Leopard, the Mac OS counted space the same way as DIX does. DIX has not yet been updated to use Snow Leopard's counting method.

Prior to Snow Leopard, a nominal 320GB hard drive always showed a formatted capacity of about 298GB, because 1GB was considered to be 1024MB, 1MB was 1024KB, and 1KB was 1024 bytes. 1024 x 1024 x 1024 = 1,073,741,824 bytes per GB. Divide the nominal 320,000,000,000 bytes on the drive by 1.073741824, and you get about 298.

Snow Leopard shows the same nominal 320GB drive as having a capacity of 320GB. It counts 1GB as 1000MB, 1MB as 1000KB, and 1KB as 1000 bytes. 1000 x 1000 x 1000 = 1,000,000,000 bytes per GB. Divide the nominal 320,000,000,000 bytes on the drive by 1.000000000 and you get 320. Drive makers all use this system because it makes their drives sound bigger.

The important thing to remember is that the drive is exactly the same size no matter which way it's counted. But it is confusing that DIX doesn't count the same way as SL. It makes DIX less useful than it used to be.

19 GB of space used on a brand new drive straight from the factory is perfectly reasonable.

Because I'm not in front of your computer and haven't used Disk Inventory I can't tell how accurate it is nor would I worry about it. HD space is so inexpensive these days ($.16 per GB) if you wanted to replace the drive you have with a high quality 500GB model. You can install it in less than 10 minutes. I'm basing this on a 500GB Seagate Momentus 7200 RPM drive for $79.99 from MacSales.com.

So at the end of the day I would say your computer is acting 100% normal and to start enjoying it.

Your welcome.