How much should I consider allocating if I wanted to go a bit beyond a "Live CD" experience but not quite as far as making it my A-1 Linux?

My first experience with Linux (or Unix) and GUI together was LinuxPPC on a 603e Mac clone. That was on an 8GB drive (that used to call a RAID server its home, incidentally). Then I had OS X, versions 10.1 and 10.2 on a G3 iMac (40GB boot drive), followed by OS X 10.3 Jaguar on a G4 Dual 1.25 MDD. The power supply died on that -- a $300 item when you can find one with the right pinouts.

In x86 land, on this Lenovo M55p (80gb boot, 1GB RAM, Windows XP Pro SP3 as the primary installed OS), I've sampled GNOME and KDE thanks to Wubi installs that were 15Gb and 25Gb, respectively. I also have an IBM Thinkpad T54 (1.25GB RAM, also 80GB boot) onto which I've installed Ubuntu 9.04.

I understand that Debian has no Wubi counterpart; that it runs strictly on X-ready file systems (Ext2, Ext3 come to mind as examples with which I am vaguely familiar). I have also heard, often enough to start believing it, that Ubuntu and its K & X variants are derived from Debian. I get the impression, however, that for a decent install of it, somewhat more than 15 or 25 GB may be required. Am I right in thinking asmuch?

BZT

Depends on what you want, Debian on my Eee 701 with lxde only takes up 1.6GB. I have seen people with a window manager only using around 500mb.

Even on my desktop with a ton of games installed my system is right at 12GB.

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Base install of Debian (no GUI) is under 400mb. A full install with Gnome and everything is under 4gb.

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Really? Must have gotten Debian confused with FreeBSD then.

Okay, so I have a 40gb external HD, already formatted Ext3 because it's got Kubuntu 9.0.4 on it. Using Kubuntu's installer on their LiveCD, I "gave" the whole drive to the install, yet I see it has so far taken up barely 3.3Gb for the OS, the KDE4 GUI and a few extras. Would it be possible to put Debian alongside this Kubuntu without repartitioning? Can the "daddy OS," in a manner of speaking, share the same space with one of its "kids" without hogging the boot order or confusing/messing up GRUB?

Is there a way to do this?

BZT

I would recommend a separate partition for your Debian install.

It's not recommended, it's obligatory

Yes, you'd need to make some space for a Debian install. Anything between 5-8 GB will do just fine. Use Gparted or similar to decrease the Kubuntu partition and so create room for Debian. Leave it empty, the installer will find it and ask you if it's to be used.