GF of course wants some place with a great resort(rooms on stilts in the water is a huge plus for that)
Ace summed up your options nicely. It seems your girlfriend wants the "total" resort feel to her trip, day spa, massages, guided tours etc. you can scratch a lot of places off your list. Like Ace stated, the Marshall Islands and Micronesia (Pohnpei, Chuuck, Kosrae etc) doesn't have the infrastructure in place to support western style tourism. You come out here because of adventurism and hardcore diving, not western style tourism.
If stilted and air conditioned rooms are a need, of course thats Bora Bora and the Maldives all day, but be prepared to pull out your wallet.
If the south pacific is a must, Yap is the place to go.
I simply care about diving, good food and not crowded beaches.
If you want a combo trip that'll keep the GF happy, you could expand out to the more traditional "coral triangle", which will have a different look and feel to the south pacific.
It'll be cheaper as well. there's more flights going to manila than to majuro. you can fly from ATL to MNL for half the price.
Bali is a bit overrated IMO, but might fit the bill for your needs....everything seems crowded to me, but then again, i live on an island with a thousand people. Theres touristy stuff to do like temple tours in the day, nightlife and bar hopping at night.
Thailand is a total blast. Bangkok is a fun cultural place, plenty to see, eat and do. Head down to Patong, and its the wild wild west (for fun..), plus you can sneak away for a day trip diving while the GF hangs out. Its very easy to make friends while you are there.
You could also do the Phils. Boracay is the party central island. I was pleasantly surprised at how good the diving was considering how developed the tourism was on this one island.
There's tons of other islands to consider with a lot of natural beauty like bohol, palawan etc. the flights into Manila are cheap, and the local island hopper flights are cheap as well.
Just some food for thought if you decide to expand your search a bit further.
Good luck and good diving!