I just took a quick look and I did not see what install method or version you are using, but I used iPCOSx86 10.5.6 Uni Final and my quad core works great. Threads was one of my best stats on xbench. I have been up and running for a while and have not done any type of install in a while, but, as I recall, the only time I saw any issue with cpus= was with a patched kernel, ie, not updateable. My updateable install works fine. Check out some of my older posts, see if that helps.

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Intel DG43NB, ICH10, Ethernet Intel 82567V, Audio Realtek ALC888VC, nVidia 9400 GT, Core 2 Quad Q8200 2.33 GHz, 4GB, SATA WD 500GB, DVD Sony DRU-865S
EFI v9 courtesy iPC OSx86 10.5.6 - Updates via Software Update!

God made man knowing his new creation would not live up to Gods' own standards, so God gave him a loophole, ignorance. Now man can exist in their imperfect state and God won't have to squish him. So, if I am ignorant at times, chill. God made me that way.