I normally want about 500 images on a card so it will handle a full day of shooting landscape and wildlife images. That won't cover all days, but does a pretty good job.
The 75 mb file is uncompressed - it looks like closer to 45-50 mb with lossless compression. That means I will need 22-26 gb for 500 images. I'll be rounding that to 32 GB.
The other issue is speed. This is a factor at two points - downloading from your card and when you are filling the buffer with a long burst. Downloading is easy - a fast card makes downloads a lot faster. A faster USB 3.0 card reader can help too.
At 6 fps with a grip, you are talking about 300 mb/sec plus whatever processing time is required. Whether you are using a 30 mb/ sec or 60 mb / sec won't make a lot of difference - it still takes time to clear the buffer once filled.
I'm planning to use Sandisk Extreme or Extreme Pro cards. I'll use a 32 GB CF card in slot 1. I can probably live with a 16 GB SD card in slot 2, and I may use an 8 GB Eye-Fi card on occasion. I'll have a 16 GB CF card as a backup for slot 1.
BE careful of counterfeits - buy only from known sources and be careful with anything that is too cheap or off ebay.
I don't think highest speed cards or extra large cards are worth the money for most situations. I want to stay close to the best card - not pay the premium for the best cards.