How To Monitor My Web Server's Performance

I've recently found (last 6 weeks) that the performance of my client's websites on my trusted host's servers isn't how it used to be and/or how it should be. The download seems to take way much longer than before. So far it hasn't resulted in a significant drop in average pageviews per unique visitor but it might do if things continue. The host claims a number of attacks on the mailserver and an unusually high load at one point in time. Above all it's just annoying - isn't reliable server performance what can be expected from a reputable host?

Here's the question: what's the best way of monitoring online how the web and mail servers are performing so that I can take this report to my host and urge them to take (more) action? Ideally I'd like to compare this with a seperate web server that I use for another client. I don't mind spending a little bit of money but high subscription fees are not in my budget.