Not all quad cores are created equal either - be careful. This chart is extremely helpful in learning how these different machines perform:

http://www23.tomshardware.com/cpu_2007.html

The 3D Studio Max benchmark is probably the most relevent to concerns about rendering speed.

I almost bought one recently with the Core 2 Quad Q6600 - for about $1100. At that price, you could have 8 of them for $9000 and use the last $200 for beer.

The Q6600 is the first one that is much faster than the Duos (47 seconds versus 69 seconds), and it's cheap. Above that the prices go up fast, for not much additional speed. The top of the line (at the time that chart was made) QX6850 was 38 seconds for the same render. I'd rather not pay an additional $1000 to $9000 just to save 9 seconds from that render. Or even for 9 minutes off of a 47 minute render. I'd rather have 8 Q6600's and render with 32 cores (if I had a Poser that could use a render farm.)

In the end, I chose the Athlon 64 X2 6000+ (117 seconds) with a 500G drive, 2 G ram, and a good hardware graphics card - all for $730.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)