You'll probably get better suggestions if you provide more details about what you are looking for.
Just about any modern system can be used for development, though you'll want more CPU and Memory. I'm doing some development at home on the couch with an inexpensive laptop. It's not as good as a big multi-monitor system, but: couch! ;)
A good graphics card is useful for DX, but not so necessary for other apps. Multiple monitors and a touch screen are both very helpful. Ideally I'd want An SSD for speed and a large drive to host Hyper-V images for testing. Especially for a laptop you
might want TPM chip so you can bitlocker your source code.
As much as you can afford ... I'm running an i7 3960X at 4.6Ghz with 32GB RAM And 1.5TB of SATAIII SSDs and VS 2012 is very slow, worse than VS 2010 for large scale projects. XAML editing is still a VERY painful experience and big productivity killer
... we often end up just editing anything XAML related via a separate editor like UltraEdit. Sadly VS 2012 is still a native 32bit application so we're restricted to 3GB private bytes.
But we've put a stop to Win8 projects based on lack market penetration. Rumors of Windows Blue will be out this June, so you may want to hold off and see where that goes, but sadly it's much of the same so I'm not very optimistic about future
development in either platform.