You can get the sysutils package (http://sysutils.rubyforge.org/) for
some quick and easy access to that sort of thing.  The downside is
almost all of them are very gems dependent, so its difficult to
install them standalone.

Sys/Admin (User & Group information, as well as some other stuff I
don't use as much)
Sys/Uname (Has some or all of the stuff you want, and is quite usefull)
Sys/Host (just hostname stuff)
Sys/Cpu

--Kyle
#Example follows.  Enjoy!

require 'rubygems'
require 'sys/uname'
require 'sys/host'
Sys::Uname.release
=> "2.6.18-8.1.15.el5"
Sys::Uname.uname
=> #<struct Struct::UnameStruct sysname="Linux",
nodename="lxwrk1.trisalesco.com", machine="i686", version="#1 SMP Mon
Oct 22 08:32:04 EDT 2007", release="2.6.18-8.1.15.el5">
Sys::Uname.sysname
=> "Linux"
Sys::Uname.version
=> "#1 SMP Mon Oct 22 08:32:04 EDT 2007"
Sys::Uname.nodename
=> "lxwrk1.trisalesco.com"
Sys::CPU.processors
=> [#<struct Struct::CPUStruct flags="fpu vme de pse tsc msr pae mce
cx8 apic mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse
sse2 ss ht tm pbe up cid xtpr", fpu=true, fdiv_bug=false,
vendor_id="GenuineIntel", cpuid_level="2", cache_size="512 KB",
bogomips="4800.05", wp=true, coma_bug=false, f00f_bug=false,
fpu_exception=true, model_name="Intel(R) Pentium(R) 4 CPU 2.40GHz",
processor="0", cpu_mhz="2399.026", stepping="7", hlt_bug=false,
model="2", cpu_family="15">]