I have a website that has approximately 300 users and I get about 1000 hits per day. Due to an upcoming event next week, there is a possibility of all of a sudden getting between 20 and 25 thousand users per day, with between 60 and 70 thousand hits. I currently have 2 CPU on a semi-dedicated hosting. My host is aware that there could be a bump in users and said that he could quickly up the CPU as needed. However, there's also an option of prepaying for additional CPU. As the 20-25K users would be purchasing access to my site, I definitely want that aspect to go smoothly. Is there anyway to tell whether I should purchase access to the additional CPU?
1) A screen shot of my usage of the last week (the spikes are nightly database/file backups and won't increase with the new users)
3) My "purchase access to the site" script takes about 2 seconds to run as it's interacting with my payment processor.
4) Each page that the user accesses requires a check via php to see if they're logged in, and 1-2 trips to the database: typically page loads are under 5 milliseconds.
If I'm being neurotic and shouldn't purchase more CPU, then that's fine for someone to tell me so. However, if this seems like a case where I should make the purchase, it would be great to know what number of additional CPU cores would be appropriate.