As you play more and more, the value of legendary marks will diminish. With this in mind, it is best to simply give in and upgrade gear as you go instead of hoarding them.
If you plan to save, the best piece to buy with your legendary marks if you are at a low light level is the 280 ghost shell.
Okay, so we are past hoarding and now into spending marks with disregard. In order to maintain a solid piece of legendary gear, it will need to be infused.
Infusing does not change the roll on the gear. Infusing will increase the stats of the gear slightly to scale depending on the increase but will not change the type of stats. Infusing will give roughly 80% (plus rounding, which seems to be normal rounding, so above .5 rounds up, below rounds down, and .5 rounds to the nearest even number - to note though, .5 is impossible to reach when multiplying a whole number by .8) of the different in light (which is to say that a difference of 100 yields +80).
Infuse often, and make sure that you save some of your higher light blues in order to raise up gear which may be quality but not quite high enough to use.
Exotics can be infused, even with blues. Legendaries can be infused, even with blues (or exotics, or legendaries).
Infusing will open an interface (so you can click it and then look at the interface before use). The interface will hold a set of that same slot gear which has a higher light level than the piece to be infused. Then you must hold x on the selected piece to dismantle it (this destroys the other piece) and the upgraded light level is shown once complete.
The only thing that infusing increases is stat numbers and light level.