Web & DB Servers

I'm wondering what the better solution is:

Apache/PHP & PgSQL on one physical server with 100% performance  (ie. 1200
MHz, 2GB RAM) or have Apache & PgSQL each on a server with half the
performance (600MHz/1GB RAM each).

I'm especially curious in the way long running queries affect the L2 cache
performance in the mixed enviroment. What if Apache & PgSQL are fighting
over the L2 cache making it's large size (in case of the new .13 micron
CPU's from Intel) rather useless compared to the "dedicated solution", where
each CPU runs only one application and can use the cache better? Maybe I'm
missing something here completely, so I would appreciate any
comments/suggestions. Is there a noticable performance gain in splitting the
server in two?

Thanks!

-Zavier