A lot of people have observed and I have tested it and it works for me, that the sweet spot is letting the statement post with under 10 percent utilization, but not zero. In your case, that means let the statement date come with a balance of not more than 50 bucks, but not zero. Do that consistently for a few months and you'll likely(no guarantees, nobody can say for sure) see some positive stabilization in the movement of your scores due to the card.
I don't know how far you are from buying a house, but having only one open revolving credit line is not ideal. 3 is the magic number I've seen repeated(I don't know for sure, I do know that with 2 open accounts, I continue to have "not enough current accounts" listed as a negative against my score, but I just added another 2 so we'll see if that helps). With your scores, you can probably get approved for a decent no annual fee unsecured card somewhere, but I'd encourage you to consider a credit union. However, that could whack your score in the short term just on the hard inquiry alone. A safer in the short term way would be to get another secured card, which would avoid the hit from the hard inquiries(assuming you apply at one of the many places that doesn't do them for secured cards), but still might get a hit with a new line of credit open(might not though). A good card, though it doesn't graduate, is the Visa Platinum Rewards from SDFCU(apply by phone to avoid hard inquiry, I've seen reports the online app results in a hard inquiry).
What you can do at this point depends a lot on what your time frame for buying a house is. If you're looking at a year out, then you have a lot of options. If you're looking at next couple months, that's a lot dicier and I'd be hesitant to offer any advice as I've only personal experience in moving my score in longer time frames.
