UPS

depends what components you are using

system specs please

I forgot to mention ill also be plugging in a 23" lcd i guess that uses up considerable power as well

Head over to eXtreme Outer Vision, their PSU calculator sizes your system's PSU (free version) and UPS (pro version, $10 lifetime)...

From my understanding, your UPS wattage doesn't exactly have to be the same or higher wattage as your PSU but it obviously has to be able to handle your actual usage. For example, you have a 750W PSU but your peak usage load may only be around 400W so you'd only need a 400W+ UPS. I'd recommend getting a Kill-a-Watt and seeing what your actual peak usage is but keep in mind you may upgrade your PC more often than your UPS so leave room for expansion. I'm also not aware of typical UPS efficiencies but keep in mind a 400W UPS may actually max out at 350W because no PSU or UPS is 100% efficient.

Just looking at THIS UPS at newegg for examples sake, it tells you it's rated for 600W and can handle the following:

Entry-level PC with 19" LCD (150W) - 15 min.
Midrange PC with 19" LCD (300W) - 7 min.
Performance PC with 19" LCD (500W) - 3 min.

Keep in mind though that you'd only be using upwards of 500W if you continued heavy work or gaming while the power was out. The Kill-a-Watt would give you a much more realistic understanding of how little of that 750W PSU is actually being used during light-to-moderate usage.

From my understanding, when the power is on the wall outlet is filling the UPS battery at the same time the PC's PSU is drawing from the battery. They run this way to avoid any lapse in power when the house power goes out. So to answer your other question, no. A 300W or 400W UPS can't supply 600W when plugged into the wall (nor would you want it to because if the power did go out at that elevated wattage your UPS would be pointless).