The memory_limit has nothing really to do with the amount of data you need to store. Thats disc space. The memory_limit is the amount of RAM required to parse some of Tiki's code. Tiki is a massive application and needs more than the default 8Mb, 12/16/32, are all good values, the bigger out of the three being the best.
First question before you continue to use hostrocket is to ask them What is your PHP memory_limit set to? If they respond 8Mb and they wont increase it. Dont bother continuing and look at the TikiFriendlyHosts page.
Whilst Tiki attempts and seems to work in some 8Mb environments, It will get to a point where one day, it will just stop. especially once you have started to load up some "content"
TikiWiki Project Admin