Windows 7 or vista..?

I would suggest windows 7...Its faster on a dual core processor and most of the new laptops are based on a dual core processors...(xception netbooks)
Its also better than vista because it has lessser memory requirements(RAM)
So go for one that has windows 7 .........

But for people with Vista there's no reason to upgrade to 7, unless you're having major system performance issues, which are more than likely related to the hardware you're on and only going to be slightly mitigated by switching to Windows 7. Basically if you had problems before you will have problems after on the same hardware period. Considering the cost of buying a retail Windows 7 upgrade or full version, it's cheaper to buy a new machine. Sad but true.

Home Premium Upgrade can be found on eBay for about $80.
Not that expensive.

personally i would stay away from the upgrades. if you can do a fresh install you going to be far better off.

My train of thought when a customer is deciding weather to go xp or 7 is what are they going to use it for. you have to make sure that your devices have the right drivers that work with 7. Plus there is some older software packages that simply wont work.