The total output wattage thing is pretty much meaningless. I've been using a Shuttle Silent-X 250w PSU for yonks and that thing was always the subject of 'can I use this graphics card with my dinky psu?' For modern computers the total output of the psu is not as important as the amperage it can supply on its 12V rail(s). On your PSU there should be a sticker that looks like:
This one is the famous Silent-X PC40. The little psu that could. I had an nVidia 8800GT and an Intel E6750 running on this thing and it was a charm (still works!). The label tells you all you need to know about what it can do.
+12V x 16A = up to 192W of the total output can be supplied as +12V power which is what all the power hungry components in your PC like to suck on. That includes your cpu, graphics card, all that good stuff.
-12V x 0.3A = up to 3.6W of the output can be supplied as -12V power. This is something of a legacy since hardly anything uses this or -5V lines anymore. Back in the day it was old ISA bus cards which mostly disappeared a long time ago.
Then we get to the capacity which can be supplied at low voltages:
+5Vsb x 2A = up to 10W of the output can be supplied as standby power which is always active, even when the computer is in (you guessed it) sleep states or hibernation. It keeps certain things like the keyboard powered so that a keystroke input can wake the system up.
+5V x 19A = up to 95W. Typically used to power the PCI bus and storage disks etc.
The +3.3V line in this psu and many others is regulated down from the +5V line. This one can supply
+3.3V x 18A = up to 59.4W of the total output to stuff like the USB bus, RAM and supply power over SATA.
The combined output of the +3.3V and +5V lines however cannot exceed 105W. The total combined output is 250W so the psu cannot supply the maximum rated current on all of its supply rails at the same time. Theres some juggling going on depending on the power requirements of your computer. Most modern PCs lean heavily on the +12V rail(s) so when you look for a psu you should really be looking at how many +12V rails the psu has and how much amperage it can supply on each +12V line.