Do the Aluminum iMac models have PCI slots or expansion bays?

No Aluminum iMac models have PCI slots. Officially, no Aluminum iMac models have an extra expansion bay either (no formal bays beyond those occupied by the hard drive and optical drive by default). If expansion is a top priority, and cost is a factor, you might be better served with a used model from the Mac Pro or Power Macintosh G5 series.

However, as first noted by site sponsor OWC, the 27-Inch "Mid-2010" models -- the iMac "Core i3" 3.2 27" and "Core i5" 2.8 27" -- have an extra 3 Gb/s Serial ATA connector and power for an optional SSD, but if one is not installed at the time the system is purchased the connector cables and mounting bracket are not present. Consequently, this isn't really an expansion bay per se but it is possible to install a 2.5" SSD in addition to the hard drive if an SSD is not installed initially it's just a bit of a "hack."

Likewise, all "Mid-2011" models -- the iMac "Core i5" 2.5 21.5", "Core i5" 2.7 21.5", "Core i7" 2.8 21.5", "Core i5" 2.7 27", "Core i5" 3.1 27" and "Core i7" 3.4 27" -- have an extra 6 Gb/s Serial ATA connector and power for an optional SSD that can be considered an expansion bay as well.

It is assumed that the education-only iMac "Core i3" 3.1 21.5-Inch (Late 2011) does not have this effective "drive bay" for a 2.5" SSD, but this is speculative. If you have additional information, please share.

What are the capabilities of the optical drive provided by the Aluminum iMac models?

All Aluminum iMac models have a slot-loading 8X dual-layer "SuperDrive" -- capable of burning double-layer DVDs at 4X and single-layer DVDs at 8X. The drive can burn CD-R at 24X and CD-RW at 16X as well.

Do the Aluminum iMac models support dual displays? What is the maximum resolution for an external display?

All Aluminum iMac models support at least dual displays.

Apple reports that the "Original" and "Early 2008" Aluminum iMac models are capable of supporting digital resolutions up to 1920x1200 and analog resolutions up to 2045x1536 on an external display while simultaneously driving the internal display at its native resolution.

The "Early 2009," "Late 2009," "Mid-2010" and 21.5-Inch "Mid-2011" and "Late 2011" Aluminum iMac models are capable of simultaneously driving the internal display and up to a 30-inch external display (2560x1600). The 27-Inch "Mid-2011" models are capable of driving three displays -- two external displays up to 30-inches (2560x1600 each) in addition to the internal display.

Do the Aluminum iMac models support 802.11n?

Yes. Apple's specifications formally note that the systems provide "built-in AirPort Extreme wireless networking (802.11n)." As first discovered by the now defunct HardMac, the current "Mid-2011" Aluminum iMac models can support even faster 450 Mbit/second 802.11n using the MIMO wireless standard.

For the wireless capabilities of all "recent" Macs (G3 and later), please refer to the AirPort Capability page from EveryMac.com's By Capability Mac specs listing.

Also see: What is is 802.11n? How is it different from 802.11b, 802.11g, and 802.11a?