Is it possible to upgrade the processor in the Aluminum "Unibody" Mac mini models?

No. As first determined by MacminiColo -- and confirmed by hands-on observation -- the processor in the Aluminum "Unibody" Mac mini models is soldered to the motherboard and it is not possible to upgrade it after purchase.

Photo Credit: MacminiColo (Aluminum Mac mini Processor)

However, it also is worth mentioning that it was possible to upgrade the stock processor in the Mac mini "Core 2 Duo" 2.4 (Mid-2010) to a 2.66 GHz processor for an additional US$150 via custom configuration at the time of purchase and is possible to upgrade the Mac mini "Core i5" 2.5 (Mid-2011) with a 2.7 GHz Core i7 (I7-2620M) processor for an additional US$100, but again, only at the time of purchase. As requested by readers, EveryMac.com also documents these custom configurations as their own models.

Does the Aluminum "Unibody" Mac mini have a PCI slot?

No Mac mini models -- Aluminum or Polycarbonate -- have a PCI slot or other internal expansion slot.

If expansion is a top priority, and cost is a factor, you probably would be better served with a used Mac Pro, Power Macintosh G5, or Power Macintosh G4 model. Site sponsor Operator Headgap Systems specializes in refurbished, and often heavily upgraded, systems capable of running both Mac OS X and Mac OS 9 applications.

How is the SD Card slot different on the Aluminum "Unibody" Mac mini compared to SD Card slots on earlier Macs?

The SD Card slot on all Aluminum "Unibody" Mac mini models -- inconveniently located on the rear of the system -- supports SDXC SD cards. These cards look the same as earlier SD cards but support a theoretical maximum of 2 TB of storage.