Considering the two options you suggested, I would choose the second one - install Windows7, and then install Ubuntu on separate partition(s).
Before you start installing anything, you need to understand the concept of a Linux system structure - it is different than the one from Windows family systems.
Not digging deep into technical details, you might use as many as 4 partitions for Ubuntu alone, each serving a different purpose. For a start, you might use just 2 partitions:
- The root directory (
/ ), which would contain the system itself, along with boot loader files in
/boot and user's home directories in
- A swap partition. Its role is similar as swapfile's in Windows family systems, just that in Linux it should be a separate partition.
Concerning the sizes of those partitions - I'd say that using around 2-3 times the amount of RAM you have for swap partition and around 10-15 GB for
/ would suffice for a start. The size of
/ partition is a matter of personal choice, depending of how much software do you intend to install, how much files do you want to store on your Linux system etc.
There's a short article about planning the partitioning of your hard drive when installing Linux systems on your computer. Basically, it might be justified to use as many as 4 partitions for one system - in addition to the two mentioned above, you might want to put
/boot (the boot loader files, etc.) and
/home (user files) each on separate partition. Take a look at this article for more information; you might want to google some more about this issue.
After some research, it will become clear to you that the idea of Ubuntu and Windows7 sharing the same partition is unfeasible. Not only it is hard to enforce Linux to use NTFS filesystem (although Ubuntu should be able to do so), but also because those two operating systems have very different approach to hard drives, partitions and system devices.