I also agree that the main limitation is not processing power, but RAM. Unless your site is hideously badly coded, processing requirements are going to be relatively minimal, but RAM on the server-side for PHP is going to be 8 MB minimum per concurrent session, perhaps a good deal more if your page is also accessing a mySQL database. 8 * 128 = 1024 MB of RAM, and the cheapest virtual dedicated servers are still limited to 1024 or 2048 MB, with up to several hundred MB often taken up by overhead (depending on how well they're set up). So you could hit the limit. I would guess the same is true of Perl.

Short version - if you use a virtual server, you probably want the 2 GB and not 1 GB. (and yes, I do use PHP in addition to Perl - which I use for a site is dependent on what the site needs to do)