How to Define Max Concurrent Users?
 

We bought vBulletin 3.8 and had it installed and were running it on Yahoo hosting (bad idea). Our site probably maxes out at about 200 different visitors in a 10 minute span and about 1000 very active users. This traffic brought the site to a crawl periodically on Yahoo hosting, although, you could have 5 minutes of flawless, high speed performance, followed by 30 second page loads for the next minute, then back to OK.
As we're looking for a new hosting provider, we see different discussions of concurrent users but I don't see it defined precisely - does that mean the number of people loading a page at the same time, the number of people reading the forum at the same time (of which maybe only 10% are actually hitting the server at any given moment) or the number of people who were on the forum in the last 15 minutes?
With that amount of activity, anyone want to suggest our needed server requirements? The site is somewhat image intensive.

thanks in advance!