3 questions need answers please

Regarding your first problem with hard drive space and adding in new stuff, I would imagine it's to do with Windows allocating a bit more space for the page file (which generally seems to mirror the amount of RAM you have). A page file is a little portion of space on your hard drive set aside by Windows, so if you run out of space on your physical RAM, it spills over into the page file, meaning your computer doesn't instantly fall over when you go 1mb over your physical RAM.

Regarding your PSU: Firstly, it's fine for your system, mate of mine runs essentially the same rig but with a 450W PSU. But, to calculate how much power you'd be using, simply look up and read reviews for the stuff you have, look at the power usage (most reviews have a power usage section), add these together, and bob's your uncle. Manufacturers generally state a "minimum PSU" for the whole system, rather than saying how much power the component uses. The most important parts in your computer are generally run of what is called the "12v Rail". You can calculate how many watts the 12v Rail can hold, by looking at the specification sheet of the PSU and looking for the amperage of the 12v Rail, then multiply the amperage by 12 (for the 12v Rail). For instance, if the 12v Rail has 40amps available to it, then it can sustain about 480W, so after reading the reviews of your components, you can essentially see that your processor will take about 100W, meaning that you have about 380W to play around with, baring in mind other components that may use the 12v Rail.

If you are worried about heat in a GPU, take it out, blow some compressed air through it, clean the fan/s, then test again for heat. Ideally, you don't want to underclock/undervolt, else you wouldn't have gotten that card in the first place! What temperatures are you getting, by the way? The best way to see if your card is running OK is simply to Google "Nvidia 560 maximum temperature", or something along those lines for you, there will be multiple threads of people asking the exact same thing, so there's bound to be an answer in there somewhere!

look on the power supply label look at the 12v wattage and amp rating. cpu (77w). per online info the max voltage of the card is 150w. and 75w for the pci slots. llooking at 25.1 amp draw for max load for that set up.
if the unit has 2 rails that 18amp or larger your fine. the down side is check how many pci video card power plugs that are the unit. most good gaming power supplys will have two.
underclocking the gpu will lose you some frame rates...with gpu use msi afterburner set up a fan profile to make the gpu fan spin up under load. most newer gpu the firmware and drivers try to keep the fan at low speed to keep the noise down..this causes some of the smaller cards like yours to run hot. also with gpu if your case has spot for fan on the bottom of the case put one there have it blow cold air into the case and over the gpu.
with windows the more ram you add the larger the swap file is made. if you have a ssd drive and not a hard drive turn off window swap drive and if you have another data drive put your windows restore info on the data drive.
there few good tools and web info on setting up a ssd for windows 7/8 to free up space.