Welcome to cruise-chat.
Actually, my normal advice to a newbie is to use an agent and don't try this at home. The price is supposed to be the same no matter if it is from an agent, on-line, or on the phone with a PVP (Carnival's phone people). Same goes for upgrades. Speaking of upgrades - don't get caught up in it too much. 99% of the time all it is is the exact same size and type of cabin, but on a higher deck. It isn't going to be a move from a standard cabin to a suite, especially during the peak summer cruise period - which is where August falls.
We usually bring $100 per cruise day, thus $700 for a 7-night cruise. We'll take $150 to $200 ashore but never use it all. Note this doesn't include cash outside the cruise itself. I'll have another $300 just-in-case money for airports, cabs, etc. We also take a credit card ashore - each of us with a different one.
It's good you are planning on doing the Sail & Sign...it isn't an option!
The only places cash can be used on the ship is in the casino or for any extra tips you give. Carnival defaults to a set tipping fee being automatically charged to your Sail & Sign account. It is currently $10 per person/per day. We usually give extra in the form of cash to our cabin attendant, dining room staff, room service, and favorite bar tenders and bar waiters.