Best option to build credit when buying a computer

Hi all,

I'm in the market for a new computer and also have been building my credit from nothing in the last year.

I paid off what was on my credit report and started with three secured credit cards, keeping the balance under 30 percent and paying them off every month. I now have about a 650 credit score, but it is stagnant and not getting any higher. I'd like to get it up to at least 700, so I can plan towards buying a house at some point.

My car is paid off - two of the three cards are no longer secured and raised my limits, while the third I need to cancel, but I've heard that also effects your score?

At any rate, since my score is stagnant, I'm trying to find another way to raise it. I do need a new computer.

I went to Costco today and, for better of worse, got their credit card under my executive membership - they offer basically a no interest plan on a big purchase, but because it's on a credit card versus financing through Costco, I'm not sure that that would raise my credit, rather, would it lower it by having a revolving balance for the first time?

My other thought was financing directly through the retailer - that would operate more like a loan, correct? I honestly could buy a cheaper computer outright, or pay a nicer one off in a couple of months, but just want to use the opportunity to bump up my score, since I won't be taking out a car loan or similar any time soon.

What do you all think? What is the best way to use this purchase to potentially bump up my score?