So when I play a card, I understand that I pay the gold associated with it and if I wish to play a card from a different house, then I must increase the cost by 2 gold.

What I am unclear on is EXACTLY where do I reference the affiliation, is It only the shield at the top, or is it the letter that is just below the art, on the right side of the card.

If that little letter has no relevance, other than a simple "filing number", could I then add cards like Street of Steel from the Stark deck, which is neutral, into my Targaryen deck, thereby increasing the cost to 2 gold?

Just getting my head around a couple of the rules.

Also, are plots, for the most part, interchangeable b/w houses?

House affiliation is determined ONLY by the shield in the top right-hand corner of the card. The letter next to the filing number has no relevance, other than what Starter Deck the card belongs to in the Core Set. Neither does the text of the card (for example, an event that says "House Stark only" is still considered neutral, without a House affiliation, because there is no shield in the upper right-hand corner).

A neutral card has no House affiliation at all. Since it has no House affiliation, it will never be subject to the "out-of-House" penalty of 2 gold. So you could move Street of Steel from the Stark Core Deck to the Targ Core Deck and still pay 0 gold for it (i.e., no increase in price at all for the neutral card).

Plots are entirely interchangeable between Houses, unless they specifically say "House X only."

Thanks ktom

Are there any particularly great plots for Targ?

Thanks ktom

Are there any particularly great plots for Targ?

While "Fury of the Dragon" and "Mad King's Legacy" (for example) are house targaryen only, one of the most popular Targ plots is "Threat from the North". It's not House Targaryen specific, but it synergizes so well with their existing "burn" (strength reduction) effects that it is more often played out of Targ than any other house.