There are many guides out there regarding how much swap, which are usually wrong. One common myth is that you should have double the amount of swap than you do ram. This is old advice that was useful advice for windows 98.
Here are some rules to go bye:
- At a minimum, you need 1g of combined ram + swap space.
- For hibernation, you need at least as much swap as you do RAM.
So, if you have 256MBs of ram (the minimum needed for ubuntu), you should have 756MBs of swap. (But, you should also consider buying some more ram, because Ubuntu will run slow, even if you are running Xubuntu.)
As the amount of RAM you have goes up, swap, of course, becomes less necessary. With 2g of ram and 2g swap, my installation has never used the swap, ever. Usually, my memory usage sits around 0.5g in use and 0.5g of cached files.
Here are some links with info about swap.
https://help.ubuntu.com/community/DiskSpace#Swap has a great overview of what swap is.
https://help.ubuntu.com/community/SwapFaq has tons of info about swap, everything you could want to know. The only problem is its suggestion of having swap be double the size of your ram, which for most people, is way too much.