Other things to consider:
Delta has a 50% transfer bonus from MR->DL. Delta also has a purchase/transfer miles promotion. Transfer miles from your account to another account and get a 100% bonus. Ends up costing 1.1 cents per mile. Transferring 100k miles to your GF would be the same as purchasing 100k miles, and saves about $175. Delta also has a 40% MR points rebate for miles transfers in chunks of 50k miles. Transfer 50k MR to miles, get 20k MR points back (several weeks later). If you have a longer time horizon this is the best option.

I'm not saying Delta is the best place to put your miles, but explore it as an option. Before you do it, I'd recommend trying to book several 'dummy' award trips to check potential destinations, cost, and availability. It's probably the easiest to bank large numbers of miles, but it will be tougher to redeem and require more effort/flexibility to get what you want.

To maximize DL miles I would do this:

1. Register for the 50% bonus for first time transfers. Note that this is only for first time transfers, so make sure you do this first and don't register for the other promotion yet. http://dmn.delta.com/ace_offers/junemr/
2. Transfer 25k points of your 75k to DL for the 50% bonus for 37.5k.
3. Register for the 40% points rebate offer: http://dmn.delta.com/ace_offers/julymr/ . Make sure you do this after the first transfer. Since the first offer is only good for first time transfers, this one should kick in automatically.
4. Transfer the remaining 50k MR points to DL for 50k miles. In 6-8 weeks you'll get an e-certificate from Delta good to get 20k MR points back.

*the promotion is on the Delta side, so you won't see any mention of it at AMEX when transfering points. All you need to do is register, and when you make the AMEX transfer Delta will pick it up on their end
**Your 40% points rebate probably won't come back in time to be able to move those to CO.

After that you'd have 152.5k DL miles, and if you used the bonus to transfer them to your GF you would pay $1,650 to get 150,000 miles for a total of 300k in your GF's account and 2.5k in yours.