Matthew,
Good questions. Your new MacBook should be plenty fast to run the tracks. There are a few things you can do to increase performance.
1. Increase the HW Buffer Size in Ableton preferences and definitely go over 500 samples.
2. Secondly, when you're building a new session, make sure you have all your files in that sessions folder. You can do this by choosing "Collect All and Save" under "File" in Ableton. *Note* We recommend you re-save your sessions when you alter them and keep the originals just as you downloaded them from IWL.
3. If this does not fix the issue, you can use an external hard drive to store your files keep Ableton running on your hard drive. This is the preferred way to run audio files, but even so, the files you download from us should be small enough to run on the same drive as you run Ableton. As you're library grows and as your Custom Sunday files grow with more songs and video tracks, you'll want to make the jump to an external drive. Any Firewire 400 drive will do but make sure it's a 6 pin connection, 7200 rpm, and 8 MB cache or higher. The On-The-Go drives that we sell through our store are great for this application because they are bus powered through your single firewire connection (you don't need a cord plugged in for power) and they're really small and sturdy (fit in your pocket).
4. If you're upgrading your ram on a Mac, you can get a great deal at www.macsales.com
5. Built-in sound is fine but it limits you to two outputs (left and right). So, if you're running tracks with a drummer you can put the click left and the tracks right, but that means you cant run the tracks stereo and you'll lose the stereo imaging if you do this. The Firewire 410 will give you 8 analog outputs and you'll really enjoy the difference of the tracks being stereo in your ears, and if you're house system is stereo, the congregation will enjoy it too. It's worth checking to see if your PA is stereo, and if so, make sure your sound guy pans the tracks hard left and hard right at the sound board. If you have the Aviom monitoring system, you can do the same for your ears and it will make a huge difference, and you can run the click up the middle (both left and right ears).
5. Running a Firewire 410 will not decrease performance.
6. I would not recommend running any live tracks through the 410. Just run them separately through a mixer because latency will be a big issue and there is no way you could run effects. In a live context, I would dedicate your laptop to playback only and have it do that one job really well.
Phillip