I thinking about to put a Spark to the reset and power jumpers on the motherboard on a computer.
Thinking also to put in the power led to it so i can get if its power on or not.
Does anyone see any problems with it?
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Sounds legit and feasible 
You can get a true feedback from the computer by using the led output fed into the core as an input
Yup, just make sure you use a 5V tolerant I/O line on the Core for the input from the LED. Slap a 1K resistor on there just to be safe. (Value doesn’t have to be exact, higher would work.)
I’m not sure how the reset/power circuitry works on modern ATX motherboards. It could be setup so that on one of the two wires there’s a voltage output and when the button is pressed it completes the circuit and goes through the second wire which is an input, thus signaling the computer to turn on or reset.
The other way it could work is that the input wire is being driven high (carries a voltage via a weak pullup resistor) and the other wire is connected to GND; when the button is pressed the input wire goes from high to low, which the computer notices and turns on/resets.
For the power/reset signals, use a voltmeter to see what kind of voltage is on the output (or input) sides. If it’s 3.3V you’ll be fine; if it’s 5V you should be okay, as the “high” threshold the computer’s motherboard detects should be around 2.5V or so.
In the first scenario, just hook your Core to the input wire and use digitalWrite to briefly pulse high, then low (with the default being low). In the second scenario, go from low to high (with the default being high), it also wouldn’t hurt to use a low value resistor (220 Ohms) to prevent the Core from absorbing too much current.
If I had to guess, I’d say the second scenario is most likely, as that’s how most CPU and MCU reset lines work (pullup resistors connect to the RST line and keep it high while running; if the signal on RST goes low the internal POR [Power-On Reset] circuitry goes into action and initiates a reset).
Pin 16 on the 24pin ATX connector is typically used to turn the PSU on or off, by connecting that pin to ground it. It may be possible to sense if the machine is powered on by checking if that pin has been grounded or not, another possible way to check if the device is on would be to tap in to one of the 3.3v rails on the PSU as these are only live when the power supply is in the “on mode”
You can also grab 5v power from pin 9, which provides 5v Standby power, so is live even when the PC is off, this pin can only provide a very small amount of current.
http://forums.extremeoverclocking.com/attachment.php?attachmentid=142848&d=1262854051
The 5V standby rail can normally provide at least 1A; the label of the PSU should tell you. There’s a completely separate switching supply designed to feed this rail. 
(I just took an old ATX power supply and made it into a 15 port USB supply, which is a great use for them if you have one laying around!)