Hi everybody!
I just received an email from an officer where she stated that my new classification is IR-6, and that approval letter was sent two weeks ago. I never received anything though.
Anyways, not to confuse you completely, my national passport will expire in a couple of months, and a new one is in process for over five months already (they say it takes 4-6 months). My I-551 stamp will expire in two weeks, so I don`t think it is possible to either get GC (I`ve never done biometrics for I-751), or my new national passport will be issued before that.
What should I do? I just went for fingerprints for N-400 on 7/28, so I don`t think I`ll take the test within two weeks. Will I be ok with I-551 in expired passport or I`d have to stamp my new national passport as soon as I get it?
Any other suggestions and advices are welcome.

This is not a problem, and LPR is not required to keep passport current, many become citizens long after their foreign passport expires. The I-551 stamp is considered valid for 1 year from issue, it does not expire when the passport expires.

You could have let it expire and just apply for an American passport after becoming a citizen.

What if I travel overseas? Will I be ok with two passports?
I am planning on keeping my original citizenship, so I need that passport, too

One passport or the other, you could have problems if you have both in your possession at the same time.

Hmmm...Are you sure? I was told that, if travelling to my country of origin (USC required to have a visa to enter), I have to have both passports. When leaving US-use US passport, when entering country of origin-use that one, leaving -use foreign passport, and when entering US-use US passport. Anybody knows for sure? And, I guess, for travelling with my foreign country, I need to keep both I-551 stamp and passport valid all in one, correct?

See the Dual Citizenship FAQ for a discussion of this and other issues regarding what to do if you're a citizen of two or more countries at once.

The issues can become complicated, and depend very much on the laws of your other country.

US law doesn't care about any other citizenship(s) you may hold. But a US citizen must present a US passport to US officials at ports of entry. You may carry another passport if it's been legally issued by another country, but you aren't required to show it to US officials, and it's probably most expedient not to voluntarily bring it to the attention of US officials unless asked (but don't hide it or deny it if they ask about it).

Some other countries have laws similar to the US laws, in that they require their citizens to present ONLY their nation's passport when dealing with their officials. If you're from one of these countries, you'll need to carry both passports. Show the US passport when dealing with US officials, and the other passport when dealing with officials from that country.

Some airlines and other non-government officials may be confused by multiple passports.

Some other countries may prohibit carrying two passports. Some may prohibit being a citizen of another country while you're a citizen of their country. This is where it can get complicated.

Yes, I was told exactly the same thing by Consulate of my country of origin-show another passport only when they ask to show. I guess I`ll know for sure how it works when I do it.
LucyRich, thank you very much for your knowledgable posts, you make this site a great reference