Viscosity - the manuals say that 5W30 is preferred at all temperatures. They say you can optionally use 10W30 if the ambient temperature in your area doesn't go below 20 degrees F.
Synthetic vs Conventional - Synthetic oil is more resistant to breakdown at high temperatures, and has better flow characteristics at low temperatures. Conventional oil is cheaper. You can find endless arguments on the internet, either way, but those are the primary benefits of each. (I use synthetic in all my cars.)
Brand - I've tried a lot of different brands, and haven't seen any big difference between them. Personally, I think the difference between one brand of synthetic oil and another is far less significant than the difference between synthetic oil and conventional. So I generally get full synthetic oil, whichever major brand is the least expensive.
Keep in mind that you will be replacing your oil on a regular basis. This lets you try out different brands with each oil change, and if you keep accurate records and your usage doesn't vary, this will tell you whether there are really any differences. For example, people told me that the expensive European Castrol Syntec would burn less oil, but I tried it and it wasn't any different from the other major brands.
FYI, here are some recent prices for various brands of synthetic oil (5W30 and 10W30 except where indicated):
Single quarts of oil:
Mobil 1 $6.99 Autozone, $6.27 Wal-Mart
Castrol Syntec $6.99 Autozone (including the 0W30 from Germany), $6.27 Wal-Mart (which did not have 0W30)
Pennzoil Platinum $5.97 Wal-Mart
Royal Purple at Wal-Mart - if you have to ask how much it costs, you can't afford it
5-quart jugs of oil:
Mobil 1 $26.00 Wal-Mart
Castrol Syntec $25.00 Wal-Mart
Pennzoil Platinum $23.00 Wal-Mart
Valvoline Synpower $22.50 Wal-Mart
Quaker State (Q) $18.00 Wal-Mart
An off brand (I forget the name, I assume it's their house brand) $17.77 Wal-Mart
Six-quart cases of Mobil 1 at Costco and Sam's Club are usually more expensive, per quart, than the 5-quart jugs at Wal-Mart, although they may be less expensive than single quarts there. IIRC they are currently around $34.xx.
Sometimes the oil manufacturers offer promotions that are worth considering. Quaker State recently offered a $10 rebate on five quarts of synthetic (Q), which means it was only $8 for five quarts.