There's a couple of links below but if you google it you will get a lot more. The numbers sometimes conflict.
There is a difference btwn improvements and repairs. While I would be cautious in improvements, in general, I would make ALL the repairs that I could. For example, say you need a new roof, which would cost $5,000. The Buyer is likely to demand considerably more than that off your sales price. They don't want to take any pricing risk and they also really don't want the hassle of repairs. If there is a major repair such as a roof needed, subconsciously the buyer tends to think that you haven;t really taken care of the place, even though that may not be true. They then tend to discount the overall offer.
Also, certain needed repairs, like a roof or peeling exterior paint, are very, very visible. The buyer may not notice that it needs a new roof (but it will come up during inspection) but the house feels tired and dated to him without the buyer really knowing why.
That being said, before I sunk any significant money into repairs, I would have a realtor or two take a look and give me an opinion. You do tend to get your best price if it sells quickly, and a quick sale is hastened by good condition. Not everyone would agree, but I would get any major repair that I was contemplating done before listing or offering the house for sale. I would tend to limit my improvements to fresh paint in neutral colors where needed, professional cleaning or a lot of elbow grease, and really sprucing up the yard. those things will make the house look well taken care of. Good luck and you have come to the right place for info!