a few things you might want to look for when deciding (based solely on my own experiences);
-ease of reading information (some of the smaller watch styles aren't the easiest to read at depth).
-nitrox compatible (you'll want it later).
-user changeable batteries
-good backlighting/illumination. Having to shine your dive light on your computer while night/low light diving is a little inconvenient, and can be a bit blinding until your eyes readjust.
-water activation...one less thing to worry about. I have seen divers forget to manually turn theirs on until they're at 80 feet...whoops.
-ease of retrieving relevant data. What do you want to make sure is on the main screen and doesn't require searching for? For most it is your no-decompression limit, ascent rate, O2 limit if on nitrox. There's a ton of other retrievable info (water temp, elapsed dive time, max depth, etc.) Just make sure what you want is where you can see it easily.
-personally, I like having audible alarms. I have a tendency to push my ascent rate a little if ascending from decent depth (much more careful last 30 feet) and being reminded when not looking at my computer is nice.
-PC downloadle isn't a must, but a great way to keep your logged dives.
A few of the more affordable computers my friends like are the Cressi Archimedes, DiveRite Nitek Duo, and the Suunto Gekko (although a few have griped about Suunto's extra conservative algorithm). My personal experience has been with the Oceanic Veo 250, which I love. It has all the relevant data in an easy to read format, diver replaceable batteries, audible alarms, allows for a slew of diver set safety features, an owner's manual that explains the computer's capabilites in an easy to read format, and it is a computer that you could grow into; it's nitrox compatible and has a ton of features, many of which you won't initially use. The price varies if you want your computer in a console with a compass and SPG (if not integrated), or a wrist with just the computer. I have the quick disconnect 3-gauge console, which new was around $600, but I just bought a lightly used one for a friend for $300...mint condition.
Once you find a computer you think you may want, go to the manufacturer's website and look over the specs/capabilities of that computer. I also find it helpful to download the owner's manual while there to review the computer's operations. You also may want to come back to ScubaBoard, type the computer of interests name in the search engine, and see what others have to say on that specific one.
I know, it's kinda like asking what flavor ice cream you want when there are hundreds of choices...where to begin, right? Just find what features are most important to you, which computers offer that and are in your price range, read the manufacturer's specs (and maybe even browse the manual), then come back here, type that computer in the thread search, and see what others have to say. You'll eventually narrow it down and make a good choice.
Hope this helps a little. Happy Diving.
Kevin