well, we've got both A64s and P4s for our linux dev workstations and while code(java) is run with databases, we also do a little multi-tasking like some firefox(browsing) , xmms(music), skype(voice chat), evolution(mail), konsole all autostarting as soon as we login on our Linux desktops.
From our experience, we find the multitasking of A64 to be equal if not better than the P4s with HT. Particularly, its very snappy when miniming the current window and selecting another window. Maybe got something todo with better implementation in the kernel or maybe the 64-bit stuff, I'm not sure which one. Note that the JVM is 32-bit emulated on the A64s, though it decreases performance < 1%. All other apps are 64-bit and the distro is gentoo(single compiled image rsynced to multiple machines).
So, CaiNaM's statement that "i've had both s754 and s939 now, and i voted intel as while a bit faster in some apps, and most games, a comparable intel doesn't give up that much, an my experience while running multiple cpu intensive apps on winxp has shown the a64 is clearly lacking." could very well be M$ buggy/bad/poor implementation of task switching/cache/page/swap handling on the A64s.
Just my 0.02
