I get this question a lot from the Why-Not-WPF-Crowd so I thought I would post it out there.
.NET Framework 4.0 – link
| 32-bit | 64-bit | Mac OS | |
| Processor | 1 GHZ | 1 GHZ | N/A |
| RAM | 512 MB | 512 MB | N/A |
| Disk Space | 850 MB | 2 GB | N/A |
Silverlight 3.0 – link
| 32-bit | 64-bit | Mac OS | |
| Processor | 450 MHz | 450 MHz | 1.83GHz |
| RAM | 128 MB | 128 MB | 128MB |
| Disk Space | 4 MB | 4 MB | 7 MB |
Sky Cliffs 