I don't think anyone said that a 2.8 X2 was 'much slower than a 1.8 C2D'.
What is true though, is that looking at the Anandtech review of the E6300 and E6400, is that the 2.13ghz E6400 w/2MB L2 on 1066FSB was a shade faster then the 2.6ghz X2, and just a hair slower than the 2.8ghz X2. The 4MB C2Ds (E6600 and above) just run away with it, let alone the newer ones with 6MB L2 / 1333FSB (Wolfdale E8xxx's)
Given that the E6300/6400 came out well over three years ago (and they weren't top of the line then, either), the even-older X2 Athlons are pretty worn out to be used for gaming with a new high-end video card.
Toyota is the one who's telling everybody that the X2 at 2.80GHz is slower than a Core 2 Duo at 1.86GHz, which is not true. Its like if the same advantage that the K8 had over the Netburst architecture would be the same advantage that the Conroe had over the K8 which is not true.
if you think his 5600 X2 is remotely as fast as your Q6600@3.2 even in games that dont use more than 2 cores then you are very mistaken. turn off two of your cores and lower your cpu to 2.0 and you will still equal or in most cases outperform a 2.8 5600 X2.
The X2 running at 2.8GHz is quite competitive with any Conroe Core 2 Duo running between 2.16GHz and 2.2GHz depending of the gaming scenario, with the slight edge going to the Core architecture. The Athlon X2 64 aka K8 architecture is slighly less efficient compared to the Conroe architecture, it was much faster than the Netburst architecture in which its Dual Core incarnation running at 3.20Ghz which was able to match the K8 running at 2.0GHz, so how suddenly the K8 which was able to smoke the Netburst architecture now runs much slower than Conroe?
http://www.anandtech.com/showdoc.aspx?i=2802&p=9
In HL2EP1, the Conroe 2.40GHz is slighly faster than the Athlon FX running at 2.80Ghz which is slighly faster than the Core 2 at 2.13GHz, and the Core 2 at 1.86Ghz is slighly faster than the Athlon X2 at 2.40GHz.
In Battlefield 2, The core 2 at 2.40GHz is faster than the FX at 2.80Ghz which is slighly faster than the Core 2 2.13GHz. The Core 2 1.86GHz is slighly faster than the Athlon X2 at 2.60GHz.
"The processor landscape has been changed once more thanks to AMD's extremely aggressive price cuts. The Core 2 Duo E6300 is a better performer than the X2 3800+ but is also more expensive, thankfully for the E6300's sake it is also faster than the 4200+ and the 4600+ in some benchmarks. Overall the E6300 is a better buy, but at stock speeds the advantage isn't nearly as great as the faster Core 2 parts. In many benchmarks the X2 4200+ isn't that far off the E6300's performance, sometimes even outperforming it at virtually the same price. Overclocking changes everything though, as our 2.592GHz E6300 ended up faster than AMD's FX-62 in almost every single benchmark. If you're not an overclocker, then the Athlon 64 X2 4200+ looks to be a competitive alternative to the Core 2 E6300.
The E6400 finds itself in between the X2 4200+ and X2 4600+ in price, but in performance the E6400 generally lands in between the 4600+ and 5000+. Once again, with these 2MB parts the performance advantage isn't nearly as impressive as with the 4MB parts (partly due to the fact that their native clock speed is lower, in addition to the smaller L2 cache), but even with AMD's new price cuts the Core 2 is still very competitive at worst. If you're not opposed to overclocking, then the E6400 can offer you more than you can get from any currently shipping AMD CPU - our chip managed an effortless 2.88GHz overclock which gave us $1000 CPU performance for $224. "
http://www.anandtech.com/showdoc.aspx?i=2795&p=14
In Quake 4, The Core 2 at 2.40GHz is faster than the FX at 2.80GHz by 6fps, and the FX 2.80GHz is faster than the Core 2 Duo 1.86GHz by 20fps which is slighly faster than the Athlon X2 at 2.0GHz.
In Battlefield 2, the same scenario repeats again, the Core 2 at 1.86GHz is slighly faster than the Athlon X2 2.60GHz by 1.5fps.
In HL2 EP 1 same story happens, but this time the Athlon X2 2.60GHz is slighly faster than the Core 2 Duo 1.86GHz.
Etc etc etc, in the end, the Athlon X2 at 2.80GHz offers the performance of a Conroe 2MB CPU running between 2.16Ghz and 2.20GHz.
"We're still waiting to get our hands on the E6400 as it may end up being the best bang for your buck, but even the slower E6300 is quite competitive with AMD's X2 4200+ and X2 3800+. If AMD drops the price on those two parts even more than we're expecting, then it may be able to hold on to the lower end of the performance mainstream market as the E6300 is not nearly as fast as the E6600. "
But in the end, at my opinion, even the Conroe at 2.40GHz can be a serious bottleneck to a HD 5850 regardless. A Wolfdale Core 2 Duo at 3.0Ghz or more is more balanced thanks to it's improved IPC. But the i7 750 is a perfect match for it.