The quality of the render only depends on the rendering application, not on the CPU.
The speed of the render most certainly depends on the CPU.

Poser 7 can take advantage of multicore CPUs, Poser 6 and lower can't.

As for Intel/AMD - I've always gone the practical route, and until the Intel Core 2 Duo CPUs, that meant AMD. 
Now I'm not so sure. As far as I can see, you get about the same CPU power per dollar with AMD and Intel (after AMD lowered its prices for the AM2 range).

I would not make the decision based on the CPU alone. Well, my own preference goes towards a fast quad core machine (at the moment that would mean an Intel QX6700 CPU) but there's also mainboards, chipsets and graphics to consider.

You will want to avoid ATI graphics cards. Theyr'e good for DirectX games under Windows 32 bit, very good actually, but they're not good for anything else. Nothing to do with the hardware, but ATI has never delivered decent drivers for anything that is not  Windows 32 bit DirectX.
AMD has merged with ATI recently. ATI has made inroads into the mainboard/chipset market lately. But I don't know about stability and perfornance yet.

For an Athlon based computer I can recommend an nVidia nForce based mainboard. Good performance, good drivers, for both Windows 32 bit and 64 bit, and decent Linux support.

If you decide for an Intel based system, I recommend an Intel 975X based mainboard. Those are not cheap, but they're very good and fast, and will support the fastest Intel CPUs, including the quad cores, and they'll also support more than 4 GB of RAM. Might sound unnecessary at first, but when (not if, when) you switch to a 64 bit operating system, you'll want to make use of its ability to address enormous amounts of RAM. Especially when you do 3D stuff.

(Personal experience: I run Windows XP 64 bit on an AMD Athlon64x2 4400+ with 4 GB of RAM and Vue 6 Infinite 64 bit. I've rendered scenes that required more than 20 GB of virtual memory, scenes with several hundred billion polygons, scenes that were absolutely impossible in a 32 bit OS).

For a graphics card, there's only one choice. nVidia. The 8800 series is extremely fast (and extremely expensive), when it comes to price/performance ratio you're probably better off with a 7800 GT /7900 GT. nVidia has good OpenGL support and also has good WinXP 64 bit drivers and Linux drivers. The GT models are faster than their GS counterparts, the GTX models are faster than their GT counterparts. 

My personal "dream PC"
Intel QX6700 CPU
Intel BadAxe2 mainboard (975X chipset)
8 GB DDR2-667
2x300 GB Seagate 7200.10 drives
nVidia 8800 GTX graphics card
DVD +/- RW station
Windows XP Pro 64 bit.

Which is about the fastest PC you can build at this time. Very expensive, around € 3800 in the Netherlands, including VAT.

A good midrange system would look like this:

Intel E4300 CPU
Intel 975X based or 965P based mainboard
4 GB DDR2-533 (2x2 GB)
nVidia 7900 GT graphics card
300 GB Seagate 7200.10 drive
WinXP Pro 64 bit

or

AMD Athlon64x2 Socket AM2 4000+ Brisbane
nVidia nForce based mainboard
4 GB DDR2-533 (2x2 GB)
nVidia 7900 GT graphics card
300 GB Seagate 7200.10 drive
WinXP Pro 64 bit.

These systems will have comparable performance and comparable prices. I recommend Seagate 7200.10 drives, they're slightly more expensive than their competitors, but they're both faster and more silent.

Hope this helps and doesn't confuse you even further...

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