If you do decide to install it, you can allocate as much (or as little) space as you want. If you plan on using Windows as the primary but will not have excessively large files (such as movies) stored under Ubuntu, then (assuming the hard drive is at least 100GB in capacity) 10GB should suffice for most purposes. The space does not have to be split in half (theoretically, anywhere between 20%/80% to 90%/10% will work), but you may want to stay with the flash drive until you can decide whether to install it or not.
Memory = short-term/working memory, effectively
Space = long-term memory, effectively
Package installation:
On the sidebar, there should be an icon for the Package Manager. From there, you can install the available packages. For GIMP, simply enter GIMP in the search box to the upper right. When the password is requested, the password requested is whatever password you set. (Do NOT disclose the password to anyone unless it is necessary, and in that case, change it after everything is done!)
Install?
If you like Ubuntu, you can install it, although you may want to read a few Linux-related books before doing this to get an idea of how to split the space. Ubuntu says it requires a minimum of 2 GB of space, although at least 8 GB of space is recommended. If you want to uninstall it, it will be more difficult (unless you use Wubi) in that there will be a free partition. This will be more risky than simply using the Wubi installer.
You should not have any problems with running Ubuntu from the flash drive, and it is possible to install packages to the flash drive, as that is where Ubuntu is located. Generally, the practical problem to flash drive booting would probably be the risk of breaking the flash drive connected to the computer physically, but flash drive installations should be fairly stable (although slower).
Wubi:
With the Wubi installation, you can uninstall Ubuntu from Windows easily and safely (and probably more easily recovered as well, in case anything happens). If you want to install Wubi, you will have to do it with the Wubi installer provided. This will also require the image file, which was hopefully not deleted.
Wubi is probably better than using a flash drive as it boots from your hard drive, which is probably faster. Also, it does not require the commitment involved with a standard installation and is easily removed. There should be no visible differences. If you want to uninstall it, uninstall it just as you would uninstall any Windows program.
Internal memory:
Since Ubuntu does not run at the same time as Windows, there should not be any problems with memory. With space, you will have to dedicate a few gigabytes to Ubuntu, but it should not have too much of an effect. I assume that your computer is fairly new (by the fact that it does run a new Windows version) and has a few hundred gigabytes of space. 10 GB for Wubi out of 200 GB space is only 5%.
Should I respond by PM to prevent cluttering this thread?