Hi,
My 2c worth: I'd say around an 800MHz+ CPU, and either Windows 2000 (with 256Mb RAM) or XP (with 512Mb RAM) should give a decent enough Amiga experience with WinUAE.
It's not essential, but can also help if the laptop has a discrete graphics card (even something ancient like a Geforce2Go or similar) with its own GPU and RAM, rather than a basic embedded effort.
And the usual stuff applies, like making sure the latest DirectX, audio drivers, video drivers, etc, are installed.
With some older laptops that use NVidia chipsets,
LaptopVideo2Go is a useful resource, allowing newer drivers to be installed on older chipsets...
- Ali