OK, I ran the Jim Burton P5 benchmark,
http://www.renderosity.com/mod/freestuff/index.php?user_id=2541
under the same conditions, plus ran some math on the results:

AMD64 X2 4400+ (2.4 Ghz, 2GB DDR Ram (480 Mhz))

400x400 (Re-use Shadow Map, keep textures in memory).
1 Core(s), 1 Thread(s): Render time = 42.35 seconds.
1 Core(s), 2 Thread(s): Render time = 45.24 seconds.
2 Core(s), 2 Thread(s): Render time = 31.33 seconds.
2 Core(s), 4 Thread(s): Render time = 32.52 seconds.

800x800 (Re-use Shadow Map, keep textures in memory).
1 Core(s), 1 Thread(s): Render time = 105.56 seconds.
1 Core(s), 2 Thread(s): Render time = 108.00 seconds.
2 Core(s), 2 Thread(s): Render time = 63.14 seconds.
2 Core(s), 4 Thread(s): Render time = 63.43 seconds.

1600x1600 (Re-use Shadow Map, keep textures in memory).
1 Core(s), 1 Thread(s): Render time = 360.52 seconds.
1 Core(s), 2 Thread(s): Render time = 358.26 seconds.
2 Core(s), 2 Thread(s): Render time = 192.74 seconds.
2 Core(s), 4 Thread(s): Render time = 182.18 seconds.

Best results, performance boost from 1 core to 2 cores:

400 x 400: 35.2%
800 x 800: 67.2%
1600x1600: 96.7%

***svdl's Intel Core 2 Duo Q6600 at 2.4 Ghz

---

Best results, performance boost from 1 core to 2 cores:
400 x 400: 29.1%
800 x 800: 55.0%
1600x1600: ???

Best results, performance boost from 2 cores to 4 cores:

400 x 400: 22.2%
800 x 800: 25.0%

Best results, performance boost from 1 core to 4 cores:

400 x 400: 57.8%
800 x 800: 93.75%
1600x1600: ???

*AMD Athlon64 4400+ X2 (2.4Ghz) 2 Cores vs. Intel Core 2 Duo Q6600 (2.4Ghz) 4 Cores

Intel quad-core advantage vs. AMD dual-core Both@2.4Ghz:

400 x 400: 39.3%
800 x 800: 57.9%
1600x1600: ???

Intel dual-core advantage vs. AMD dual-core Both@2.4Ghz:*

400 x 400: 13.9%
800 x 800: 26.3%
1600x1600: ???

*****Extrapolated based on 2 cores running on Intel Core Duo 4
core processor.

Note: My AMD processor is 1 1/2 years old, newer models at higher Mhz would fare better.

**Summary:
**Based just on raw rendering power, not taking into effect the additional multi-tasking benefits of a quad core system, the performance advantage rendering in Poser 7 of an Intel Core Duo Quad Core vs. Dual core is approx 25%. The cost differential between similarly specce'd Dual Core and Quad Core systems would need to be weighed vs. a 25% performance difference for someone deciding primarily on Poser Rendering performance.

Also note that standardized benchmarks for other 3D rendering programs show a higher advantage of quad core vs. dual core. Poser 7's multi-core rendering process could definitely be improved. With the current approach of halving or quartering the image based on the number of processes, it's just to easy to run into situations where some threads finish well before others, leaving some cores idle for the final portion of the render process.