The wattage you need is simply more then the maximum the system will draw, with a bit of a buffer. Your current 460 watt power supply might be enough (just), but you probably want a 550 watt or better power supply. [ The video card you are using requires about 285 watts of power by itself, so the most you could need would be 745 watts, but thats excessive for your requirements]
This link may be relevant as apparently the Dell believe the GTX760 mentioned in the post would be OK with this system. While I agree with poster that it would be marginal the XPS8700 appears to have very low power requirements (as is the case with many systems running newer Intel chips).
Looking at the specs for the system, it appears that it can was designed to accept up to a 225 watt graphics card (but the default system maximum power consumption was not specified), so getting an extra 100 watts on the power supply should be ample.
To answer your question about wattage and amperage - Watts = AMPS * Voltage, so more amps is the same as more watts. A system will only require the amount of amps it requires, so overspeccing the PSU won't hurt your system, only your wallet. I'd recommend an 80+ PSU as they are more efficient and generally made out of better componentary - often a smaller 80+ PSU will give more reliable power then a cheap high power rating unit.