Thank you all for the excellent advice. My arduino starter kit + extras should be arriving tomorrow from adafruit. I have gone from zero knowledge to > 0 knowledge in about 10 hours of reading, and found that many of my questions are rather trivial given the right webpage. The project 1 (temperature + gps data logger) should be straightforward. I'm going to start with a gps + datalogging shield from adafruit, and a basic temperature probe (TMP36).
My knowledge has increased exponentially since I posted, and here's where my current progress is (current knowledge-wise progress since my arduino hasn't arrived yet and I couldn't find a simulator online in less than 3 min of googling).
As far as the garden arduino (garduino, growduino) project, I found many related projects (links available to anybody interested), with several on instructables.com, and at least one in the forums here, but none exactly like what I'm going for (I have an outdoor garden, ~2500 square foot garden, and make decisions about planting and watering based on soil temperatures at different depths, soil moisture, weather forecasts, etc)
To measure the temp at different soil depths in my garden, I'm planning to get a quad thermocouple interface (it seems a lot simpler and time saving compared to configuring 3 or 4 thermocouple interfaces myself).
http://ryanjmclaughlin.com/shop/quad-thermocouple-interface-12/
I'm still not 100% sure of the differences between an AD595 and a MAX6675, but it appears to be pins (14 vs 8), and perhaps one is capable of reading very low temperatures? I have discovered datasheets, but still haven't figured out what info on them will be most useful, and I haven't started saving/cataloging them either (organization opinions welcome).
There may be better alternatives (cheaper) for reading the temperature ranges in my garden, but I couldn't find anything definitive. I plan to stick the thermocouple probes out the side of a vertically buried piece of pvc, with the probes at different depths. The arduino will sit in a weatherproof enclosure on top of the PVC, and I'll run the thermocouple wires up the inside of the pvc. I like the flashlight/torch idea for the enclosure (thanks HTH/gbulmer). I'll also use a CdS photocell to measure accumulated sunlight...
As far as soil moisture goes, I'm not sure- I have been focussed on thermocouple stuff because this seemed easier...
http://www.instructables.com/id/Garduino_Gardening_Arduino/
seems to be an easy approach (step 4).
As far as getting the temperature data to an external database, I'm still not sure what the best fit is. The Xbee seems like a great option, and if I have multiple sensors it's even better. At the moment, I'm still more comfortable with wifi (802.11 rather than 802.15). It looks like it's not trivial to connect multiple Xbees (or it wasn't at some point in the past) or to create a mesh of them. I don't recall finding the power usage differences between 802.11 and 802.15. (goal is to have something battery powered, maybe add the adafruit 6v 1.5watt small solar panel later) Also, I don't know what the price difference would be (for one or two sensors in the garden). I may go with 802.11, but I need to read more. On the Xbee upside, I could do some wireless networking with my neighbors who are about 200m away through somewhat dense forest.
I could always put an Xport and have a solar powered wired-wireless hub in my garden, but that's just asking for problems.
Thanks, Blair