You wouldn't see twice the performance increase even though in theory you have twice the processing power, however when you have multiple apache requests, and many applications demanding CPU time, that's when you'll reap the benefits of the 2nd processor.
Matthew Russell | Namecheap
I use all dual P3 1.26 for this very reason... most bang for buck.
When it comes down to it, it depends on your operating system and software.
Is your web site the kind that benefits from dual CPU?
In my case, sometimes I run a database on the same server as the web site... and if the web site has a bug in it (bad PHP or ASP) and goes into a loop, at least the other CPU is free.
I've run the same site, mirrored at two different ISP's, and identical system... one with single CPu and one with dual. Despite averaging only 20% of the single CPU... I have a lot more problems when things get borked.
Dual better for servers IF your operating system and application can deal properly with them. Sometimes wierd concurrency problems show up on DUAL that don't on single.
ASP.NET (dotNET) also uses more RAM and has limits on SESSION capability on multi-CPU systems that don't exist on single CPU systems...
One thing you should research is I have heard P4's are not really as fast as they are labeled. That is why AMD started using confusing labeling on their processors (Ie.. an Athlon XP 1700 is not 1.7Ghz, it's 1.47Ghz. They named it the "1700" because it supposedely competes with a P4 1.7Ghz).
For example, i don't believe a 2GHZ p4 is "twice as fast" as a 1GHZ p3. Anyway, I'm not really a hardware guy, I've just been told this by some other hardware guys. It may be good to do a search for p3 vs p4 on google.
One more thing - some of the p3 chips above 1GHZ have 512k cache instead of 256k. (I think these are the chips that were meant to be used in servers). Anyway, they cost a little more, but perform way better.
I'd go for a dual p3 1.4Ghz with 512k cache. That system would scream!