Ukraine is a lot closer to the American/NATO sphere than Armenia, you know.
Did you maybe mean Georgia? Though, in that case, the Georgian government wasn't exactly innocent in that conflict. They were testing how far they could go with their new western allies and ended up pissing off Russia too much.
I think anything short of a real civil war it is really very unlikely that Russia is going to intervene in Ukraine by force. Sure, they have a lot of interest in having a friendly government in Ukraine, they see it as part as their sphere of influence, there is a big Russian minority in Crimea and there are strategic interests to consider (they still have the Black Sea fleet in Sevastopol, I think). But they are not that stupid, they know they'd make the Ukrainians themselves plus the EU and US very angry by intervening, so they'll stick to diplomacy. They will probably try to influence the outcome in their favor by diplomatic/political means (and so will the West), but not much else.
On the other hand, thinking the US or NATO would (or could) intervene in case of a Russian intervention, is an illusion. Basically nobody has any interest in a conflict with Russia, that's just no option. Got nothing to do with oil or something, it's just a geopolitical conflict nobody wants and nobody is actually capable of risking at the moment.
I mean if the West doesn't intervene in Syria, because a) there is no popular support for war and b) the US military is already stretched too thin to actually fight a prolonged conflict (it's even worse with the European armies), then they're certainly not going to take on Russia. That's just unrealistic.