[Updated for Season 3]
The game is currently in flux (season 3 has only been out for a couple days), but with the boosted ambient gold gain, it appears you only need one gp/10. I've been building Philo Stone to great success. If you expect to get some significant assist-gold (you're confident in your kill lane, or you're Soraka and have a good eye for your ultimate), you could probably stand to buy a Kage's Lucky Pick, but I much prefer to only buy the one Philo Stone and start immediately after into my build (Sightstone, Shurelya's, Aegis -> Bulwark).
Again though, this is all pretty fresh. Your mileage may vary.
[NOTE: The answer below was provided for season 2. There were significant changes implemented for Season 3, and this answer is now out of date. It is provided here for history, but should not be followed.]
It depends. The rule of thumb is two (Philo Stone and Heart of Gold), and 98.355% of the games, you'll want those. If the game is going to absolute crap, teamfights started early and your team is getting routed, you may have to skip the second and start pouring that extra money into wards and an oracle's (counter to what you'd think, oracle's is a great purchase if you're falling behind and the laning phase is over).
I'd be very suspicious of my support if they got a third gp/5. The only real time I can see that being worthwhile is if you know it'll be a late, late game, and that you'll need to be a significant contributor in teamfights for damage.
Rammus' great strength comes with the element of surprise. If Rammus isn't in your face, and he isn't powerballing, then he's not scary.
First, the general Powerball gank-tip: start charging it before you actually go in to gank. It takes some time to get spinning, but lasts for quite awhile. If you just press Q and charge on in, you give your opponent precious escape-time. Figuring out the timing on this is critical.
Second, wards. Namely, destroying their's. Rammus late-game can be something like Fiddlesticks; if you can get the jump on them from a surprising position, you've really got something. Have an oracle's on your team (either you or your support) and clear out those wards. Leave them guessing.
Third, scaling MR blues. Rammus needs MR, and these provide a healthy chunk.
Fourth, effective team communication. When Rammus goes in, he goes in. He isn't getting back out. If your team is not prepared to jump in after you, you're going to have a bad time.
Fifth, make use of your early-game advantage. Rammus is FRIGHTENING during the laning phase. If you manage your Powerball correctly, and your lanemate has any CC at all, you're pretty much guaranteed a kill. Gank frequently.
Sixth, give your lanemates the kills, whenever and wherever possible. Don't ever let a kill get away, but if the opponent is certain to drop, give it to your lanemate. They can put that extra gold to much better use.
Best Answer
Seems like you are doing everything right. Make sure to keep buff, dragon, and baron timers. If you can get your whole team to help try and steal all the enemy buffs instead of just taking their wraiths occasionally.