First, congratulations on expecting!
Great idea to use your friends' SLR's for hands-on, just keep in mind that even within brands, handling can be different between models. For example, the 30D is larger and heavier than the 400D, and differs in control button and screen layout. The 400D also has a relatively quiet shutter sound (but still louder than a compact digicam).
Canon SLR's have usually been the leaders for low-noise low-light images, but the D80 is very competitive with the 30D and the 400D in this regard. You may need to shoot at ISO 800 or 1600 to get acceptably sharp images with lamps or window light indoors. Using flash is a workable but the results can be harsh-looking and you lose out on the mood of environmental lighting.
For the baby kitten photos, I used ISO 1600, lens aperture f/4 and shutter speed 1/60 for mid-morning window light. Note how the kitten's nose is sharp but her ears are not (a limitation of the shallow depth of field):
400D w/17-85mm f/4-5.6 IS USM, $1180 (I highly recommend this setup)
D80 w/18-135mm f/3.5-5.6G ED-IF AF-S DX, $1215 (longer range but no Image Stabilization)
30D w/17-85mm f/4-5.6 IS USM, $1650
Other brands: Pentax, Olympus, Sony (fr. Minolta) - interesting, but do not offer the range of lenses & accessories available for Canon & Nikon. Camera shops in major cities will have Canon & Nikon equipment for rent - not so much for the other brands.
Storage: 1GB flash cards are about $40 each - get 2 for redundancy.
Computer Equipment: make sure you have adequate hard drive space for the photos, and use software such as Adobe Photoshop Elements to edit and organize
You're right to not worry about differences between DSLR's, b/c you're making the big jump from compact point & shoot to DSLR. Starting off by shooting in all-auto mode is fine, and you'll have lots of room to experiment when you like.