If I had the money for a quad at the time of purchase I would have not hesitated. After purchasing the GPU/RAM/BOARD/CASE i had only 120.00$ left so it was either a 550 or a 720. Since I needed a few 120mm fans I went ahead and purchased the 550 and 3 fans. The 550 was installed on an Asrock 780a (NV chipset) :
http://i683.photobucket.com/albums/vv199/OvrClkr/002-1.jpg
At the time of putting the rig together I was not thinking about "if my dual will unlock or not" (and the fact that the 780a cannot unlock), I was more inclined to get 2 cores to do at least 4Ghz stable. After a weeks work of finding the sweet spot for all my voltages/frequency/multipliers I finally got a rock solid 3.995Ghz at 1.41v. Temps never reached 41c at LOAD with a mediocre Freezer 64. The performance at 3.8/3.9/4ghz was a amazing considering I paid only 99 bucks.
Then came time to upgrade the mobo/ram and use the Asrock as a backup. When I finally got the Crosshair i was eager to finish the builds ASAP and fire it up just to see if I got lucky
, changed ACC to all cores/enabled "unleash" mode/gave the core's -2 @ 3.9Ghz hit F10 and 3 seconds later the ROG poster showed my x2 as an x4. Went back into the Bios to change a few settings and i was done.
If i had 99.99$ to throw away right now I would get me a 555 and try to unlock. Those C3's are just plain awesome, even as a dual core. Gotta love AMD