Having RAM or FlashROM on the cart is no use if you can't get your program onto it. And I'd recommend flashrom over RAM, unless you're wanting the NES to have extra RAM to use.
The cart that connects to MIDI is Midines (
http://www.wayfar.net/), it uses a version of the board I designed for my Squeedo devcart (which I'll be releasing sometime, right now cable building is an issue and also firmware development with the PIC18F chip).
It depends on what you want to do with this cart, do you want to rewrite it every week or every few minutes? And do you want it to run mapper #0 games, or you want it to run your own code?
I don't know what tools and chips and stuff you already have to work with. There's several ways to make a devcart, the best way depends on what you want to do with it.