It is recommended not to apply for new credit within 6 months of applying for a mortgage. The inquiry and new TL reporting will drop your score.
As for the installment loan, if it is your only one, I would not pay it off because it adds to your credit mix.
Installment loans play a very small factor in your score. If you paid it off you would not see a very large increase.
Added, let the installment loan pay out a while. It will show your payment history over several months/yrs. This history is important as it shows your long term commitment. CC balances can be fudged, ie, min payment here, max next month, etc.
I would save your tax return for the down payment on the house.
Raising your secured card credit limits will probably not do much to raise your scores or chances of getting a loan. Having a good amount of money in your account will help though.
First off - adding any new credit card will immediately lower your credit. All that hard work to build up your score will take an immediate hit. Could be in the range of 15-20 points.
After you get by that take a nice hard look at the interest rate of that flashy new CC. That interest rate FICO wants you to pay. If you pay off your CC in full it is actually worse than if you leave a balance in their eyes. You ready to pay an interest rate to have your CC report favorable?
I say, save up your money and pay for your toys in full.