1, single sub running bridged from amp at 4ohm - With the amp you mentioned will drive a sub at 240W RMS...
2, 2 x subs running bridged from amp at 2ohm - With the amp you mentioned will push 480W @ 2ohm, divided by two subs, 240W each. Trouble with this - you need twice the volume to house the subs, and the subs need to be pretty perfectly matched (same make/model/revision) to get equal power without straining the amp - personally wouldn't bother.
3, 2 x subs running a channel each at 4ohm - Will drive each sub at 100W (with the amp mentioned), its a better plan to drive subs off independent channels (even though they are mono) if they are different makes/models. Issues are only 100W (not much to drive a sub effectively).
4, single sub dual voice bridged from amp at 2ohm - Best one to go for out the lot, will deliver the full 480W @ 2ohm, with an even impedance and won't need an entire boot.
Audiobahn kit looks nice - but isn't particularly good (looks > performance). Throw up your budget and I'll give you a rough idea of what to go for, its worth hopping onto talkaudio.co.uk for a decent thorough answer.
As always, recommended upgrades are:
3 Channel Amp (or 1x2 Channel + 1x1 Channel Amps)
Front Components (Driven off amp, with crossover)
Rear Sub (The enclosure makes 99% of the difference, so depending on the music style you like, you can go down the route of sealed, ported, bandpass etc.)