Paul,
Thanks for catching that mistake on the memory.... the Ultra-Slim uses a different smaller form factor type of memory (SO-DIMM) than the two larger versions of the DC7900 (DIMM).
A quote from the DC7900 QuickSpecs: " The HP Compaq dc7900 business desktop supports non-ECC DDR2 PC2-5300 (667-MHz) and PC2-6400 (800-MHz) memory." If I were buying new SO-DIMM memory for an Ultra-Slim DC7900 I'd go for the faster version, personally.
I corrected my post above, and here is a screen capture regarding the SO-DIMM type of memory that the Ultra-Slim needs. This also is from the QuickSpecs for the DC7900:
