Hey Fiddle,

OK, I'm going to assume that since you are using a Macbook, you have the Mac version of BIAB? Many people are using the PC version with Parellels, since the PC version has more features at the moment.

There are many ways to go, and I've spent years buying and then selling Interfaces. Also software.

You need two things to get a really great sounding result>A good interface and a good Sample sounds.

As far as interfaces go, everyone I know that uses Mac uses Apogee. The converters and Mic Pre's are top of the line.
Since you are using a Macbook, I would recommend the Apogee "One" USB interface. Very small (like a pack of cigarettes)
Awesome sound, has Line in, Mic in and a internal mic that is fantastic. Zero Latency. Cheap ....$250. You can spend up to $1000 for a Steinberg interface with 8 Mic pre's, but you won't get better sound! Here is the link, make sure you read the customer reviews,

http://www.sweetwater.com/store/detail/One/

As far as Sample sets go, it pays to use the good stuff! Although RT's are great, when you do need midi instruments, you need convincing sounds. Here are a couple of tips>

1. Native Instrument Kontakt 4>This is the premier sampler/sample library. The only issue with this is that it is NOT GM compatible, in other words, you have to manually load each patch. Not a big deal, unless you are pressed for time. Sounds are great, it is a HUGE library (40 GB) so you might want to keep in on a portable Hard Drive. Here is the link, Listen to the sound Demos>

http://www.native-instruments.com/#/en/products/producer/kontakt-4/

2. Vir2 has a Product called "VI One". It is GM compatible, which means your sound patches will load automatically. Here is the link, you can listen to the Demos>

http://www.vir2.com/4DCGI/vir2/products/vione/index.html?1198

hope this helps you,
Ed