Cheap shooting: 16GB for $96
I’ve been using the SanDisk 16GB cards since they came out in both Canon and Nikon DSLRs. They work great (I’ve had not a single problem). Right now, B&H Photo has them for $95.95 after a $90 rebate. Amazon.com @AMAZON also has them for even less (stick with the reputable vendors, avoid small vendors with prices too good to be true, lest you get a counterfeit card). Smaller capacities and/or formats are available also, the rebate covers a lot of models.
For a fraction of the price of a Nikon D3, you can insert a pair of these cards in its two slots, and have a mirrored backup of all your shots. Some shooters like smaller cards on the "all eggs in one basket theory", but I personally think 16GB is the best choice for today’s DSLRs if you know you’ll be out shooting for the day. Multiple cards also increases the risk of misplacing one (I have); keeping the card(s) in the camera eliminates that risk.
Regarding SanDisk rebates, be aware of the way the scam works when you submit the paperwork and you’ll be OK (photograph everything you send in before sealing the envelope!). See my May 1, 2007 blog entry for details.