As far as I remember, there were technical issues preventing yorkfiel CPUs (45nm quads) running on nforce6 boards, that had nothing to do with the chipset but with the reference-design from Nvidia used by most boards. The Striker Extreme and DFI didn't use the reference-design and should have gotten a BIOS-update. Unfortunately I only found a
news on a german website I visit covering the issue and the link to ASUS' infopage on the matter doesn't work anymore, but it definately says Striker extreme will get an update.
Since Yorkfield isn't mentioned in the official CPU-support pages for any nforce6 board from ASUS, but there were several BIOSes released after adding Wolfdale-Support, Yorkfield-support has either been scrapped or there were only Beta-BIOSes. Have you tried contacting ASUS on the matter?
Asrock develped two nforce6-board after the issue was fixed, while most other vendors simply went to nforce7. Both support Yorkfield,
this one ist the 680i-Board. It' BIOS could be useful.
Generally I think all nForce for Intel were hampered very muchby their incompatiblity with the next generation of CPUs to an even greater extent than boards with Intel's own chipsets. I don't know if this was because of technical shortcomings or because of licesing issues or Intel's product strategy.
EDIT: After a quick investigation for Yorkfield-support on nForce6 I found
two MSI-boards supporting both Yorkfield and Wolfdale,
one only Wolfdale and
one neither (of course being the most expensive), Abit supporting both up to C0-Stepping on
both of their boards, ASUS, DFI, XFX and EVGA supporting only Wolfdale and oddly Gigabyte supporting neither an all boards. I couldn't check for BFG and ECS, since ther sites are down.
I'm always amazed how it is near impossible to predict which boards from which vendors will support a CPU and which won't, as it seems to have nothing to do with High-end or low-end or if the vendor has a history of having good CPU-support, with one execption: If it is possible, Asrock will do it!