See :
http://www.iceinspace.com.au/forum/s...ad.php?t=88553
Things to bear in mind - driver and sofware compatibility with Win 7 - esp the 64 bit version. Don't assume - check - you may need upgrades - some new hardware even (eg usb hubs - usb/serial adapters).
For astro work - if its going to be acquisition only you do not need much grunt/memory (you could even get away with a netbook - some bargains to be had there as they are no longer hot tech), for processing much more grunt is needed, modern laptops are pretty quick CPU wise - but their HDDs are not - consider an SSD upgrade, that will make it fly...as will lots of memory if you are in 64 bit windows land. You may want a second large screen and some form of pointing device also if you go for the full monte machine - processing on a small screen is a pita.
USB ports - you will want plenty (so no, not an ultrabook)
You may want USB3 for future proofing?
Will it get outdoor use or be pampered, is battery life important?
Watch out for Win7 - it tries to be "smart" - looking for new networks, doing housekeeping (updates/patches/scans/defrags) when it thinks it is a quiet moment (like 1am) and quite capable of doing wierd things to your imaging run with one of those helpful activities or to save power - by switching off your usb controller or NIC, and all these are active by default and buried deeply compared to XP (no I am not a Win 7 fan).
Just a few thoughts. Budget is also an issue - best value is away from the trendy ultrabook and latest models. A full size 2011 model from a reputable maker in the $500 range will suffice for acquisition. You might have to go up to $1000 or even $1500 for the processing platform you will not hate.
Other areas to consider
Optical drive - DVD-RW or do you want BluRay?
Screen - size/finish/resolution/form factor (widescreen for movies)
Will you play games - if yes which