Q8300, 8GB Ram And It's Not Enough

I'm currently running a very large (vBulletin) forum on a single server. Up to now, I've been upgrading as the forum grows, tweaking what can be tweaked and at the moment, for about two hours every day (during peak time) the CPU maxes out and the webserver starts dropping connections. This isn't a massive problem at the moment, you just have to refresh more at these times. However as the traffic grows, this will only get worse.

I'm looking at my future upgrade options (the site shows no signs of slowing growth).

As far as I can tell, I have two options:

1. Upgrade to a more powerful single server

- a. Core i7

- b. Dual (two CPUs on one board) Core2 Quad Xeon

- c. Dual (two CPUs on one board) AMD Opteron

2. Two server solution, one server for MySQL, other server for memcache, xcache and webservice.

The former option is, obviously, much less expensive. Though i7 is incredibly expensive, it will offer a significant improvement. However, I'll most likely be facing the same problem in 6 months to a year...