Sony is the answer. Yes, right now Canon and Nikon are the big brands. But every time Sony has decided to enter a market segment, it becomes a dominant player. It supports thousands of lenses (many extremely good) from the Konica and Minolta lines, and theyr'e much cheaper still because there's less demand than for Canon and Nikon gear. Also, Sony actually makes the sensors that Nikon uses, so in terms of image quality you are only limited by your own skill.
Have fun, you will quickly figure out that shooting is much the same as it was with film - except that you can be less frugal with exposures. Because your memory card may easily have room for 150 shots, you can keep shooting until you're pleased with your composition. Also, being able to adjust ISO (f.k.a. ASA) every shot gives you a lot more flexibility. No more waiting to switch that roll of ASA 200 after 36 exposures to a 400, and having to wait till that was shot to make another switch.
The bigger learning curve will be your workflow (a.k.a. "the way you do stuff"). I strongly suggest you shoot in RAW, rather than JPG. There's a lot of threads on this forum discussing the pros and cons. Put extremely simply, RAW is the data the way the sensor saw it, and JPG is a highly compressed version of that. If you shoot RAW, you're going to want to find a software with which to work on the files. I use Adobe Photoshop CS3, which comes with Adobe Raw Converter. It also comes with a program called Adobe Bridge, which is a nice file browser and organizing tool.
Like sorting negatives, you will need to figure out a way to sort you files. It's a highly personal process, here's mine: I have two major folders, My Images and My Pictures. My Images is for my creative work, whereas My Pictures is for friends and family snaps (and thus accessible to my wife for making albums, email pics to others, etc.)
I will create a folder for a special occasion, like a ski trip we were on for four days. All RAW files go into a sub folder. I will work on the RAW files (deleting bad shots, cropping, color correcting, exposure settings, etc) and then save them as JPGs so my wife has access to them. Sometimes I have images that I like a lot, and do more than a little work on (B&W conversions, dodging and burning in layers, etc). I save the working files as PSD files, and put those in a separate folder as well. Then I also save a JPG from the worked-on file for the family picture collection.
So my tree looks like this:
My Pictures
-2008 02 Ski Trip
--2008 02 Ski Trip RAW
--2008 02 Ski Trip PSD
I also keep a general dump-all file for a particular time period. Q stands for Quarter, so all shots not part of anything special from Jan 1 to Mar 31 get dumped into the Q1 folders.
My Pictures
-2008 Q1 Pictures
--2008 Q1 Pictures RAW
--2008 Q1 Pictures PSD
For my creative work, I do it slightly reversed. I shoot a lot of RAW files, but don't bother to convert most of them. Once the files have been worked on, Bridge shows them with the changes, though the beauty of RAW is that with one click I can go back to the way the camera shot them. So that means my first folder within My Images will hold the RAW files, and a sub-folder contains the few JPGs I'm going to create.
So my file tree looks like this:
My Images
-2008 01 Veene Street Junkyard Shoot
--2008 01 Veene Junkyard JPG
--2008 01 Veene Junkyard PSD
There's also a Q folder into which I dump creative shots that I took while at certain events with my family. So:
My Images
-2008 Q1 Images
--2008 Q1 Images JPG
--2008 Q1 Images PSD
Oh, and then you MUST maintain a regular back-up system. Either copy your folders to another external hard-drive, and/or burn them on to DVD-ROMs. I know many people who burn their RAW files directly to DVD as soon as they unload them from their memory card, so at least they'll always have the digital negatives if everything goes kaplooey. But I suggest you back up all your files regularly, not just the RAWs.
Have fun, come back here to ask lots of question. Post pictures. And don't forget to help comment and critique other people's work as well, it will help you think about your own work.