Having completed a tour with my bike (my first road bike) I can now make a few comments on it and my other equipment, and pass on some of the many tips people have generously shared with me along the way.
The Mercier Galaxy is a good first road bike, and a good touring bike for its price. I bought mine on bikesdirect.com for $496 and it has since been discounted to $396. The price is right! In addition to taking mine on tours, I've also been commuting to work on it.
The Galaxy's frame is all steel, so it's not light, but it's also nothing that will fall apart on you.
If you are looking to buy a bike online, you do, of course, have to know your frame size. This can be a little tricky if this is your first road bike; I won't go into the details here.
The bike comes "mostly" assembled. All you need to complete the installation is a set of Allen wrenches and a tube of grease— just put grease on every metal-metal interface before you put the parts together. I put my bike together without adult supervision in a couple of hours. (The manual is both unhelpful and unnecessary.) Having a second person around during assembly is helpful. The only tricky part, I would say, is getting the derailleurs adjusted right. I don't have a good sense for visually figuring out how the derailleurs should be positioned, so it took me a few edit/ride cycles to get it adjusted to something usable. I imagine that if you took the bike in to a professional (or really, anyone who knows something about bikes) they would be able to fix this sort of thing for you in no time at all.
I installed the following parts for the tour. These were pretty quick changes: most elements took under 30 minutes to install (first time trying). The Galaxy is nearly ready to go on a tour.
Tires: Continental Ultra Gatorskins. Around town I'd mostly been riding around on 25mm racing tires. For the trip, I installed the Ultra Gatorskins, which have a Kevlar strip in them so they are more resistant to flats (I made it through the tour with no flats). I used 28mm tires since we were traveling loaded. The Gatorskins are heavier but having the extra peace of mind is invaluable. In case you want to install still wider tires, the brakes and frame do even fit 32mm tires, so you do have that flexibility.
Rear cassette: Shimano 11-34 8-speed cassette (HG40). The Galaxy comes with an 11-30. Having that last gear be so much lower really helps on steep grades. (With these gears, Redwood Gulch, a 21% grade at its steepest part, turned from something I dreaded to something manageable.) Often, with a 34-tooth gear, one needs to make sure that the rear derailleur can actually reach that far. The derailleur that comes with the Galaxy is just fine, in fact, though I had to do a bit of fine-tuning so I could access the last gear. You will need some more specialized tools (not just Allen wrenches) to replace the cassette.
Brakes: Kool Stop Salmon brake pads. These brakes are nearly silent and perform well in the rain. There's not much more to say about them! Buy a set already.
Bike computer: Sigma BC1606L. It's light and does the job. A single coin cell powers it for months. Additionally I borrowed a GPS unit for the trip, but closer to home I don't usually need a GPS.
Pedals: Shimano pedals, and Shimano M086 shoes (mountain bike shoes). I also have a matching pair of road bike shoes (R086). The mountain bike shoes have treads on the bottom so that the cleat is partially recessed; the road bike shoes do not, so they have a totally smooth bottom except for the cleat. If you intend to race, then every gram matters, and you want the road shoes. You also want the road shoes if you enjoy the feeling of being afraid of slipping and falling on your back every time you take a step on asphalt or tile. Maybe it makes you feel more alive. Otherwise… just start with mountain bike shoes.
Fenders: SKS Race Blades. I wasn't able to find fenders before I left, so I bought these in the middle of the tour. When riding in the rain, getting a thin stripe of mud down the middle of my back doesn't really bother me, but getting my socks wet from the splashback can be really miserable. The SKS Race Blades are nice because they attach with rubber straps, so they are easy to mount and unmount. (Also important, they go behind the front fork rather than trying to fit through it, so they actually fit on the Galaxy and other road bikes.)
Light: Planet Bike Blinky Superflash Tail Light. (I also had a cheap no-name front light that was anemic enough that I won't endorse it. Fortunately we didn't need to use our lights extensively.) Two AAA batteries power the Blinky Superflash for weeks or months of commuting.
Bags: Carradice Nelson Saddlebag and Carradice Bagman Quick Release (rear) and an Ortlieb Ultimate5 Compact (handlebars). The Ortlieb detaches easily and has a shoulder strap, so I left my valuables (camera, phone, passport) in there and brought it with me whenever I had to leave the bike. Extremely convenient. The Carradice Nelson fits a shockingly large amount of stuff, and the quick release mount is useful for commuting as well as for touring. Both bags are waterproof.
Update, Mar 2012: a year and a half later, this bike has seen about 4,500mi of riding on three additional tours, and I have no plans to retire it. I am still using everything on the list above except for the Sigma (I've upgraded to a Garmin Edge 800). More info about my recent tours here or here.