A good way to amass a BUTT LOAD OF GOLD in the 1660 era:
Start off in an area with several different countries, for example, the St. Kitts/Nevis area. Here, you can definitely capitalize on all the wars going on. You go capture a bunch of ships along with their loot, and come back to one of the wealthy ports to sell off the loot. Important: keep some of the ships, even the merchant ones. You can sell the ships for a lot of money, or you can keep them to hold your growing crew. Which brings me to my next point...
During all this ship-pillaging, you should concentrate your efforts on capturing the ships of just one or two countries. You should be "working" for someone powerful, in effect. The best is to just beat up everyone but the English. This way, you gain lots of promotions from them, and you can recruit men more easily. You want to build up AS LARGE A CREW AS POSSIBLE. Once you have at least 300 men (400 is preferred) go and start sacking cities.
The way to do this: sack every single Spanish city in the Caribbean. Start with Margarita and Trinidad, and work your way clockwise seems to be the best way for me. Capture a city and ransom it a couple times to get some made gold, then install a new governor for another country. This way, you get some crazy amounts of gold (sometimes 7000 or more), you get a lot of promotions and rewards from whatever countries you turn the cities over to, and you can keep recruiting more men in the newly-converted cities. What you want to do is make it so that there are ONLY TWO SPANISH CITIES LEFT.
Once there are only two Spanish cities left, you can easily blast your way through the main quest. You get the encounter that spawns Baron Raymondo or Marquis Montalban, and they'll appear in one of those two Spanish cities. It's best if those two cities are close together (the best is Maracaibo and Gibraltar). That way, they can't escape you. Those villains can ONLY set anchor in Spanish ports, so they're stuck there.
You get 3000 gold and a pimped-out war galleon each time you take out Raymondo. You can hit him 16 times, which amounts to a total of 48000 gold and a lot of decked-out ships you can sell. Marquis Montalban gives 5000 gold each time, for a total of 20000 gold if you hit him 4 times. Once you have the map to his hideout, you can go take him out (hopefully you still have at least 300 men) and then you get a whopping 100000 gold!
Trick is - you have to do all this with the SAME CREW. If you do, you'll easily have at least 200,000 gold pieces. Your crew is probably unhappy at this point, but as long as they're not mutinous, you're good to go.
Now, what you need to do is marry a beautiful governor's daughter if you haven't done so already. HOWEVER, make sure that she lives right near the Vera Cruz/Panama area. This way, every time you dance with her, she'll give you that Lost City map piece, and you can go grab the lost city ASAP. 50000 gold each time you do it, for a total of 200000 gold pieces. If you're quick, you can get all of them in just a couple years, which makes your crew VERY happy.
I forget what the exact amount is, but there's a certain number of gold that will keep your crew happy INDEFINITELY. The number depends on the difficulty level. I think it's something like 2000 gold per crew member will keep forever keep them from mutinying. So if you have 600,000 gold pieces, you can indefinitely keep a crew of 300 from mutinying, assuming my numbers are correct. I forget what the amount is, but you get my point.
I've actually done this a few times, including one time on Rogue difficulty level. The result was that I could play the same pirate forever and ever and ever, building up as much gold as I want. His fleet consisted of five or six Ship of the Lines, with all upgrades. He had close to a million gold, and several hundred crew. I just used him to keep capturing more and more ships and getting more and more loot. Theoretically, I could have used that same pirate until 1800 if I wanted, but I got bored at about 1698 or so (after starting at 1660).
Also, later on in the game, some of the higher ranking pirates will give you upwards of 30000 gold if you beat them, so that's another source of income.