I couldn't resist so I made some test myself. First of all, its not 100% comparable. I have same mobo: p35-ds3. Normally running RAM 4-4-4-12 but for this test I set default 5-5-5-18(4gb ddr2), I set FSB to 325 and multiplier to 8x on my E2160 to achieve 2.6ghz. Am comparing with E8400 set on 2.6ghz. He have nvidia GTX260(216) vs my Radeon 3870 gddr4.
I only tested xenosaga I because it took just 1-2min to get to second combat - same place where he also benchmarked. FPS wasn't that variable and most of the time kept stable so little diff. shouldn't matter much, what matter is some HW difference. First, its 2.6ghz 1mb L2 65nm vs 2.6ghz 6mb L2 45nm. Also GPU. Point of my test was to see if my 3870 gpu doesnt offload too much into cpu thus see if quad would be for ati radeons necessary.
I couldn't find PCSX2 0.9.5 Beta SVN 394 so I used SVN 377 that is available, I also used iso instead of dvd, but gfx and spu are same versions. Also all settings for gpu, spu and emu itself are same. But there was one problem, I have 4:3 1600x1200 lcd while author have 1920x1200 16:10. What I did was set internal resolution to 1920x1200 and used windowed mode maximized. On picture 2 u can see config, u can also see some options wasn't accessible, but I believe its same as author settings.
Under Dx10 I always saw these non-settable. My OS is Windows server 2008 x64, which is more less vista x64 ultimate without DRM and with some enhancements. It shouldn't be too different but I tweak my OS and turned some services off(but aero is on). On taskbar u also see am running minimum of apps, only google-talk and daemon tools, no antivir etc...
Now, when u check pictures u see resolution 512x448 on the top but its wrong. Picture 1 have set internal resolution 1920x1200 as shown on picture 2. when really set to default 512x448(native) in picture 3 u can clearly see difference in quality so it should be ok.
So what we see is ~10fps difference on 2.6ghz(picture 1 only!). First we must take into consideration L2 cache diff. and 65nm vs 45nm. But then again, his 65nm quad gave same fps. Rest should hopefully mean difference in GPU. OS and SVN version may also play a role. Thats why its not very good comparison but I was eager to see it anyway. Its 20% performance difference. I also cannot say final statement for radeon 4xxx series, but I thing if ati would write drivers to offload to cpu, they do it for all cards. I believe GPU made 10-15% diff.(or 5-8fps).
What am not 100% is if that is because GPU driver offloading or because my 3870 is simply weaker that GTX260 and that game would have use for stronger cards. In any case, quad is likely harder to OC and will not reach such a high clocks, so higher clocked dual vs lover clocked quad could end up same fps or even worse for quad.
My final recommendation would therefore most likely also be C2Duo for ati cards, but am still not 100%.
Side note: As for PC games, I finished GTA 4 on my e2160 @3ghz and radeon 3870. In game benchmark gave me 29.9fps. 1600x1200 around medium settings. CPU usage was around 80%. For me it was pretty smooth with occasional VERY SMALL hangs. But who knows a future...
Am still thinking about quad, but will most likely skip it. I would really appreciate if somebody could give answer/test to my previous post.