AMD Phenom II x4 955 BE
2x2GB G.Skill DDR3 1333 9-9-9-24
2x4GB ValuePlus DDR3 1066 7-7-7-20
Gigabyte Radeon HD 5770 SuperOverclock edition
1x 7200 RPM HDD
1x Blu-Ray combo drive
5x Case fans
1) How many LEDs do you think I'd be able to run off the excess power? I mean, I don't wanna go outrageously overboard, but I want to know what an upper limit would approximatly be (keeping in mind I have 5 fans, 1x140mm 2x120mm 1x92mm 1x80mm)
2) How would I connect them up to the power supply? Would I not need a negative current running the opposite direction? I saw, on YouTube a guy had rigged some LEDs up to the Molex cables running off his PSU, and that the 5v and the ground were connected to opposite ends of the LEDs, will this actually work?
3)What would happen if I had too many LEDs, and it took too much power away from everything else? I'm thinking decreased performance? Maybe failure altogether?
You are going to need to find LEDs that will run off a voltage provided by the PSU, 3.3V will probably be your best bet for LEDs.
LEDs are quite low power usually around a watt or two so you could rig up a ton of them without a problem.
If you have too many LEDs(not sure how you are planning to pull this off unless you raid every radio shack in 100 miles) your PSU would get overstressed and depending on the PSU it will either nicely shut down, try hard and hurt itself, or try hard and hurt everything around it, the first one is most likely for a decent unit.