WordPress officially needs 256 Megabyte of Memory. Even this is quite a high value, this is not announced in the minimum requirements (some parts of the software like fallback unzip libraries are needing this limit even since the 2.7 version).
Because raising the memory limit over your servers allowance can crash your blog on some servers, it’s adviseable previous to upgrade, check how much memory your server allows. If it’s below 256 Megabytes this could render your blog useless after upgrade.
Related Ticket: #13847