http://qubes-os.org/trac/wiki/HCL
The most important things from that page:
General system requirements
Minimum:
4GB of RAM
64-bit Intel or AMD processor (x86_64 aka x64 aka AMD64)
Intel GPU strongly preferred (if you have Nvidia GPU, prepare for some
troubleshooting; we haven't tested ATI hardware)
At least 20GB of disk (Note that it is possible to install Qubes on an
external USB disk, so that you can try it without sacrificing your
current system. Mind, however, that USB disks are usually SLOW!)
Fast SSD disk strongly recommended
--
Zrubi