If you can't figure out how to make similar sounds on the MO8, a laptop-based setup isn't going to help you. The problem is that you don't know much about creating sounds, not that the MO8 doesn't have much variety. Will you be able to use the MO8 to match any sound you ever hear? Of course not, but you won't be able to do that with any one piece of software either. The advantage of a computer-based setup is that you'll have easier, cheaper access to more sounds, which you can do by buying more plugins; it's the more modern version of keyboard players who used to tour with a ton of keyboards, each for different sounds.
It sounds like your problem though is that you can't find the sound that they used in song X in the presets on the MO8. Well, you could buy another hardware keyboard or a software instrument that has a preset that sounds like song X, but then what about sound Y? Using presets can only get you so far. Digging into the internals of the MO8 and figuring out how to make different sounds will get you a lot farther.
The MO8 (and any workstation-class keyboard) has a group of audio samples in memory. Some keyboards let you add your own, but these generally need to be loaded every time you turn the keyboard on, which is slow, so you want to avoid it if possible. The presets on your keyboard take one or more of the audio samples and apply various techniques used in synthesizers to change the sound, as well as adding effects (delay, reverb, and so on). For example, you might have 4 or 5 string presets on the MO8 that sound different, but they might all use the same base audio sample!
The good news is that the process is relatively similar regardless of the keyboard or piece of software you use (for the most part; a few types of synthesizers do things differently, and each keyboard/synth/plugin has its own quirks), so learning how to shape sound on the MO8 isn't a waste of time if you were to replace it with another keyboard by another manufacturer down the road.
However, you were asking about using a laptop. Well, there are a few obvious advantages: you already have a laptop (or, if you were to buy one, it's useful for more than just music), the software is cheaper than the equivalent hardware, you can change, replace, or add to the set of plugins you use without replacing the laptop hardware, and the quality is often better (for example, I have a plugin called Kontakt 3 which has all kinds of samples, including a full orchestra; there are 33 GB worth of samples altogether. Compare this to the Korg M3, which has well under 1 GB of samples in memory.)
Using a laptop, especially live, has its downsides too: wear and tear on the laptop, dealing with memory limitations (on a hardware keyboard, everything is in memory automatically; on a laptop, you need to load things into memory as you use them), dealing with multiple plugins (you generally need to load all the plugins you'll ever use for a gig/service/concert ahead of time, even though you won't use each plugin all the time, which further reduces the memory you have to use), and the best/common tools for recording audio and MIDI data aren't usually well-suited for live performance.
If you do want to go the laptop route, you need at least two things: a DAW (digital audio workstation, the software that records your audio and MIDI and hosts plugins) and plugins to make the sound. You already have the keyboard (your MO8 will send MIDI data to your Mac via USB) and a laptop. For live performance, you'll also want an audio interface: a soundcard of sorts with an interface for recording. You'd need this because in a live situation, your laptop is making the sound, not your MO8 (or, maybe they both are). Your laptop has 1/8" headphone or computer speaker outputs, and your sound board will have 1/4" mic or line-level inputs. Another advantage of an audio interface over an on-board soundcard is that they have better drivers that reduce latency: the time it takes for the sound card to process the audio and play it back. You'll notice latency if you're playing and there's a delay between when you press a key and when you hear the sound. On a Mac, OS X tends to be much better about audio than Windows on a PC, so latency may not be an issue for you.
Ok, so you'll need, at least, a DAW and plugins. Well, as a Mac user, you've already got GarageBand bundled with OS X. You can move up to a bigger and better DAW (Logic, Cubase, I think Sonar has a Mac version, and there are surely others), but as a beginner, I'd suggest sticking with GarageBand for a while until you get the hang of it and figure out what features you'd want out of a better DAW.
As for plugins, GarageBand comes with some by default, I think; where you go from there depends on what types of sounds you want to make. There are numerous free synth VSTs (VST is a type of plugin), and knowing what particular sounds you want make it easy to recommend particular plugins, free or commercial. You should also take a look at
this list; it'll point you in the right direction for some free tools.
