Not side by side (hard to do sanely in a forum)... But here's the best of my knowledge...
5500:
206 Mhz Strongarm processor (ARM4)
3.2" 240x320 front-lit screen
16 MB Flash ROM (where the base OS is stored)
64 MB RAM (by default split into 32 RAM, 32 RAM-disk for storage)
"Buzzer" (Not exactly sure on this point, but I believe it can only play a few sounds, not a full speaker)
Headphone jack (standard 3.5 mm - I think)
CF slot
SD/MMC slot
950 mAh battery
Second-hand only (most likely), probably from $150-$200 US.
5600:
400 Mhz Xscale processor (ARM5) (May be PXA250 or PXA255)
3.2" 240x320 front-lit screen
64 MB Flash ROM (32 MB: where the base OS is stored, 32 MB: where user installed programs are, and used for storage)
32 MB RAM
Built-in speaker & microphone
Headphone jack (standard 3.5 mm - again, I think that's the standard)
CF slot
SD/MMC slot
1800 mAh battery
Second-hand, maybe some new, second-hand: $200-$250 US, new: $300-$350 US
6000 (assuming 6000-L - the most common version):
400 Mhz Xscale processor (ARM5) (PXA255)
4" 480x640 back-lit screen
64 MB Flash ROM (32 MB: where the base OS is stored, 32 MB: where user installed programs are, and used for storage)
64 MB RAM
Built-in speaker and microphone
Headphone/headset jack (cellphone standard 2.5 mm)
CF slot
SD/MMC slot
Built-in wireless LAN
USB host
1500 mAh battery
New is common (second-hand harder to find), new: $400-$450 US
Note: THE PRICES ARE MAINLY GUESSES.