It won't be the 64 bit OS unless they specifically say so in their description.
There's lots of stuff on this forum about the merits of 3GB vs 4GB, the nub of which is this with regard to 32 bit OS:
1/
3GB is vastly better than 2GB.
2/
This is the contentious bit - 4GB is better than 3GB. It's contentious because you'll end up with ½GB unuseable after your Vide RAM etc. has been mapped in. On the other hand, you'll be 1¼GB better off than 2GB.
You might ask, then, whether or not you should move to a 64 bt OS. Here is very solid advice. Check first that there are 64 bit drivers for the specific laptop you're thinking of buying. This may well be tricky. For example, who'd have thought that the reknowned Toshiba didn't have 64 bit drivers for their current laptops? So be very careful there.
Hope that deals with your question.