I knew I forgot to mention something! You'd think 12 GB would be enough for that! Nonetheless, it does almost seem like a "simple" web page wants a fast machine with a Quad-core CPU and 8 GB of RAM to perform *at all* nowadays.
Given how costly RAM is for these Mac Pros (only in the last year or so have 800 MHz FB-DIMMs become somewhat affordable), 12 GB ought to be sufficient, even if it's not optimal.
An SSD would be cheaper than a RAM upgrade, and even a modestly fast one will really speed things up (virtual memory benefits from an SSD as well, and that can alleviate some of the not-enough-RAM issues, although the SSD's life might be somewhat shorter than if you didn't use virtual memory).
c