It mainly depends on what your doing. If you doing alot of mysql things, then a intel p4 would be best, unless u were thinking about a celeron and hten a athlon would be better, but in some cases the celeron woudl be better (1.3 ghz version) because of the increased cache size of that cpu over the 2.0 version of 128kbit
by the way i don't have all 4 of those kinds of servers, i stick with the dual xeon, let this be a lesson to all of you that saving money on your server specs won't help you @ all.
I used to be an all out AMD fan until about the AMD 1800+ Athlon. Thats about the time intel released its 2.4Ghz with 800FSB H/T
That processor will kill any AMD processor (besides the 64bit). I also like going with Intel because of the warranty program. Intel offers 3 years on processor and board, compared to AMD and lets say Asus which have a 1 year.
I dont have any web hosting servers right now... but I do have a nice Pentium 2.8GHz 800FSB H/T with a Intel BOXD865PERLL motherboard running 2 80GB WesterDigital WD800JD's (SATA) as a file server at my house. That thing seems to be kicking rather good.
I run a Windows server with dual AMD MP 2200's, 1.5gb RAM, and mirrored 120 SATA drives with 256mb controller cache. This seems to handle anything I can through at it.