Hi from the Foundry site:
System Requirements for Nuke
Nuke 5.1 is supported on Mac OS X 10.5 (Leopard) (32-bit only), Windows XP SP2, XP64 and Linux CentOS 4.5 (32-bit and 64-bit)
Windows and Linux
550 MHZ Pentium III or newer processor.
Windows XP (with Service Pack 2 or later) or Linux CentOS 4.5.
5GB disk space available for caching and temporary files.
512MB RAM (minimum requirement).
Workstation-class graphics card, such as NVIDIA Quadro series or ATI FireGL series, or newer.
Display with 1280 x 1024 pixel resolution and 24-bit colour.
Three-button mouse.
Mac OS X
Intel or PPC. Mac OS X (10.5 Leopard)
5 GB of disk space available for caching and temporary files.
512MB of RAM (minimum requirement).
AGP or PCI Express graphics card with at least 32MB of video memory.
Display with 1280 x 1024 pixel resolution and 24-bit colour.
Three-button mouse.
Rogue Keyframe
Feature Film Compositing
