Remember the old ecology saying, "the solution to pollution is dilution?"
You don't have enough positive tradelines. I would go for a secured card...if you don't have any baddies for the past two years see about getting a BofA "99/500" card (you pony up $99 for collateral, they give you a CC with a $500.) I would get two secured cards (you probably can't get unsecured with FICOs in the low 500s).
Keep your utilization under 10%, pay on time, etc. This will begin to dilute your bad credit with more good credit history (three current TLs, all in good standing). You'd be surprised how much you can raise your FICO by the end of the year--probably a good 20 to 40 points. It takes about six to nine months after establishing a new tradeline for the positive effects to really begin to kick in, but kick in they will.
And if you haven't bugged your current CC issuer for a CLI this year, do so.