PHP Offsite Accessing FMSA Local

I am trying to determine what kind of performance I need out of my hardware given a couple of different scenarios.

I am setting up a php site on a third party web server, so I don't have to worry about a complete lack of web presence if our connection is down in the office. (which I have already determined will require me to know what components they have turned on and off in php)

I want this site to use data coming from a FMSA server at our location with the Server being one box and the webserver for the Web functions of FMSA being another box.

As far as the servers here, which one is going to take the most heat when having the data accessed from the web? Is the Web Server going to be mostly a Processor and RAM intensive process and not so much disk, since the php pages are all offsite?

I am guessing that the FMS machine is going to be the more important one in this scenario, but I was wondering if anyone could offer some insight based on experience. I am trying to see if I can at least use one existing server for one of the tasks, rather than getting blood from a stone twice.

Thanks.