I've been playing around with P7 on my dual-core Athlon 64 running Windows x64, versus me Turion MT-40 laptop (single core), and while you see a little bit of a speed boost, it's not that dramatic running multiple threads. From what I can tell, the multiple threaded helps more with getting around the memory limit in a single thread (and thus, more complex scenes are able to render) and the render calculations rather then the rendering itself. Honestly, if you are thinking of getting a "Quad-Core" computer just for Poser, I think you're going to be wasting a lot of money for the expected return. You are much better off getting a mainstream Athlon X2 or Core 2 Duo system and saving the money.

 As far as CPUs go for a new system, in general, Core2 Duos are the better performers right now from the benchmarks I've seen on the high end (which is mostly games, which tend to be single threaded); Athlon X2 tend to be much easier to get and are better value (they've been out far longer), and holds it's own against the mid level C2D.  Either one is going to be a MUCH better performer then Pentium 4's no matter the P4's clockspeed or hyperthreading (P4 was a complete joke of a processor IMHO). I would doubt that the newest quad-core system is really going to be that much faster then a lot of the mainstream systems in Poser. Poser, being a weird animal composed of some very old and very new software technology,  always seems to hit a wall with scene complexity  that is the real limiting factor, (haven't seen this yet in P7, but I'm sure it's there) and not the hardware it's running on. It should tell you something that Vue, Bryce, Carrara, et all, can render faster, more complex scenes with their engines then Poser can on the same hardware.

While on the topic of CPUs, it should be noted, that the FSB buses (and speed) on the Athlons and Core2s are not really "apples to apples". Athlons have an integrated memory controller and thus it does not communicate with the main memory over the FSB as Intel chips do. Core2 makes up for this by having very large L2 (and even L3) caches to diminish the performance impact when it has to do memory calls through the FSB. Either way, the FSB speed is less important than it used to be.

At this time, I think both the Intel and AMD "Quad Core" designs are not worth it; I would wait until next year and see what products develop in that segment. Both companies are involved in a marketing driven technology race at this point. Intel's "Quad Core" chip is actually just  two dual core chips on the same piece of silicon, but all four cores must share the FSB bus for access to the memory; the could be a major problem for anything that is extremely memory intensive as it could cause the cores to stall out if they have to wait in line for access to the memory (something I can see a program like Poser doing). AMD's "Quad-FX" is basically  the same thing, as it is just two physical duo-core chips on a motherboard; the integrated memory controller and Hypertransport Bus helps mitigate some of the bottleneck issues with this approach but it's still hardly what I would call a true "Quad-Core" design.