I use RedHat Fedora 13. I have it on a high end 7 year old laptop.
1.3 Ghz / 2GB / 1440x1080 resolution. At work I have 64 bit Fedora 13
on a modern 64 bit laptop: Duo CPU  2.50GHz / 4GB / 1280x1024 and I
use a desktop monitor.
   They both perform well. I use Sun's, now Oracle's VirtualBox, to
run XP. VirtualBox is free software virtual machine. It is a no
brainer to install and run. I mostly run VirtualBox at work for
Outlook email client. We have Exchange mail server and to take
advantage of all the features requires Outlook newest version.
   I was using Crossover Office, (pay version of WINE, Windows
emulation) for Linux to run Outlook, but I couldn't get the new
required version of Outlook to work. So, I dedicate 1GB memory to a
virtual machine running XP/Outlook.
   I use Google Chrome (web browser) and it is very snappy; loads and
runs fairly fast. I also use Google Mail, and of course Google web
apps run real well in Google Chrome.
   I do not know of a virus that I have had in the last 13 years of
RedHat. Of course I am sure I have had them, they just do not affect
the OS/machine. The problem with that is: you can have a virus in a
jpeg image, send it to a Windows user and infect their machine.

Blake