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1. That is not called anything in the webhosting terms. All you need to run XOOPS with an arbitrary amount of XOOPS users is a MySQL database and 5 MB of space on the webserver.
If you want to have more modules, images, this and that, then you'd need more space, but 20 MB is usually enough - 100+ and you should be having no space problems at all
2. Data transfer is a maximum of what your site MAY use - bandwith is how fast it can go.
e.g. 15 GB a month is what some hosts allow. If your average page is 150 KB, then you can have roughly 100,000 page hits per month.
Bandwith is how fast each page is loading. If 20 people are on your site and load a page at the same time and each have a download bandwith of 256 Kbit/s, then for each visitor to feel that it is fast, the server will need a 5 Mbit connection (there are some packet-efficiency making it less straight forward, but let's just go with this example) to serve the visitors.
Usually good webhosting companies have a big enough bandwith to serve your visitors fast enough - but if you have large pages with lots of images or host files to download, 15 GB can easily be used.