If you want to run Windows 8 on your PC, here's what it takes:
1 gigahertz (GHz) or faster 32-bit (x86) or 64-bit (x64) processor
1 gigabyte (GB) RAM (32-bit) or 2 GB RAM (64-bit)
16 GB available hard disk space (32-bit) or 20 GB (64-bit)
DirectX 9 graphics device with WDDM 1.0 or higher driver
So I recommend upgrading your megahertz to at least 1 ghz.
32 bit: 1 gigabyte ram
64 bit: 2 gigabyte ram
Your 20 hdd will work.
Hope it helps