how much ram I can install and use on my server?

You need 8GiB for *one* guest? What are you planning to run in it? SAP!? :)

Or do you need 8GiB for multiple guests?

I suppose you are going to use a 64-bit platform because most x86 platforms can only take up to 4GiB of RAM.

Multiple guests: just another ERP and maybe Exchange. I' buying a Dell server with 2 quadcore xeon etc.., 8GB of RAM and other accessoires. But the question is: have Ubuntu any problem to manage 8GB? Thank

Thanks for your question.

I assume you're buying Xeons with EM64T, otherwise the hardware would'nt support more than 4GB per processor.

Ubuntu (as Linux in general) has no problem handling this amount of RAM. Don't ask me if there's limit configured for the Kernel-image, but if there's any, it's for sure configured to at least 64GB. So 8GB is not a problem.

Actually it depends which kernel you install. The standard -generic kernel in Ubuntu Feisty cannot address more than 4GB.

CONFIG_HIGHMEM4G=y
# CONFIG_HIGHMEM64G is not set

If you install the linux-image-server-bigiron kernel however, this has the 64G option enabled.

So in short, yes, Ubuntu can address all that memory.

Ok, I suppose the linux-image-server-bigiron kernel choice exist only for the 64bit version, because the 32bit version has made only for 32bit CPU and the physical limit of 4GB. It's right?
And on other distro, like Original Debian or ... I don't know... there's the same limit? Or they handle the 64GB without choice and tricks?

Actually no, the -bigiron kernel is also available for x86.

alan@mother:~$ apt-cache show linux-image-2.6.20-15-server-bigiron
Package: linux-image-2.6.20-15-server-bigiron
Priority: optional
Section: base
Installed-Size: 83924
Maintainer: Ubuntu Kernel Team <email address hidden>
Architecture: i386
Source: linux-source-2.6.20
Version: 2.6.20-15.27
Provides: linux-image, linux-image-2.6, fuse-module, rhcs-modules2-1, ndiswrapper-modules-1.9, ivtv-modules, kvm-modules-2
Depends: initramfs-tools (>= 0.36ubuntu6), coreutils | fileutils (>= 4.0), module-init-tools (>= 3.2.1-0ubuntu1)
Recommends: lilo (>= 19.1) | grub
Suggests: fdutils, linux-doc-2.6.20 | linux-source-2.6.20
Conflicts: hotplug (<< 0.0.20040105-1)
Filename: pool/main/l/linux-source-2.6.20/linux-image-2.6.20-15-server-bigiron_2.6.20-15.27_i386.deb
Size: 24377446
MD5sum: 27a5f5a373c53efdfea686fa7b6d0ec2
SHA1: a688b3e53363ce19d9bcd7510a02d55806dfc907
SHA256: 1dc2f3a75157da58105dc11dfcf37a73ee8f3d74c73965e0be4aa989e1edd6f7
Description: Linux kernel image for version 2.6.20 on BigIron Server Equipment
 This package contains the Linux kernel image for version 2.6.20 on
 BigIron Server Equipment.
 .
 Also includes the corresponding System.map file, the modules built by the
 packager, and scripts that try to ensure that the system is not left in an
 unbootable state after an update.
 .
 Supports high end x86 processors.
 .
 Geared toward large server systems.
 .
 You likely do not want to install this package directly. Instead, install
 the linux-server-bigiron meta-package, which will ensure that upgrades work
 correctly, and that supporting packages are also installed.
Bugs: mailto:<email address hidden>
Origin: Ubuntu