New macs use OS Snow Leopard. Leopard is a step behind if you buy a used Mac. If you have Tiger like my old Mac you can't upgrade the OS.
That's actually not true. There are three ways to upgrade from Tiger to Snow Leopard.
The first and cheapest is to simply buy the $29 upgrade disc. The upgrade to Snow Leopard will work just fine. It is, however, a violation of the EULA from Apple. The $29 disc is intended to be used by people who currently have Leopard since Snow Leopard is rather more like an update rather than a true upgrade. But like I said, it's a violation of the license agreement, so you take your chances with that.
The second option is to buy the Mac Box Set for $169. This is a stupid requirement from Apple IMO - you should be able to just buy a license for the software you need/want not be required to purchase a bundle of other software you may not need/want.
The third option is to find a copy of Leopard 10.5x (likely on ebay) and install that, then go through option number one above, but without violation the EULA.
As far as the OP is concerned, I would seriously look at the newer entry level MacBooks. They are right at about mid product cycle, so there's not going to be any updates soon, but by buying a new one, at least you know you're getting the latest and greatest when it comes to hardware and software.