Personally, I wouldn't bother repartitioning it quite yet--I'd just leave Windows 7 on the whole drive, and boot Ubuntu from a live CD a few times to see if it's something you could see yourself using on a more regular basis.
As others have pointed out, Windows 7 is happy with 20 GB or less, but it won't take long for Windows Updates, data, and any additional programs you install to start gobbling up more space. At this point, your RAM is probably the most limiting factor, followed by CPU and graphics card.
You'll want to use a low-resource-optimized variant of Ubuntu. At a glance, Lubuntu (as suggested by BJ292) looks promising. Once you start using Ubuntu more or if you want to install any additional software, then you can install Ubuntu on the hard drive. At that point, you can shrink your Windows partition with Windows' Disk Mangement utility--or, if I recall correctly, Ubuntu's installer also gives you the option to resize your partitions.
If Windows will remain your primary OS, you probably don't have to reserve more than 10-20 GB for Ubuntu. If you need to, you can mount the Windows partition in Ubuntu and save files there.