To directly answer your question, yes, if you buy an OEM version of the same edition of Windows 7 (you cannot move from Home to Pro or vice versa) then you will be able to re-install Windows. Dell are pretty good at offering all the drivers for their laptops on their website too.
However, you don't have to buy an OEM version of Windows 7 from Dell.
If you can get hold of the retail version of the same edition of Windows 7 (and there are multiple ways to do this) then you can edit a file on the disk to turn it into an OEM version - which will then happily install and register using the product key attached to your laptop.
Full instructions to do this can be found at this superuser answer.