First of all, don’t assume anything about the electrical service to the room. There could be two or more breakers assigned to the room or, on the other hand, only one that serves the entire room and other areas of the house. Find the breaker box. If you’re lucky, the circuits are clearly marked as to which breaker protects what area. If not, turn the breakers off one at a time (ignoring any that are rated higher than 20 amps) and check the outlets in the room to see if they’re still hot. When you find one that kills one or more outlets in the room, check outlets in surrounding areas. Make note of which breaker does what, and the amperage rating of each breaker (clearly marked on the breaker itself). NOW you have some knowledge.
Whether the monitors/TVs draw their full wattage or not, they’ll draw steadily when they’re on. For the sake of this discussion, let’s assume 150 watts per, for a total of 450 watts.
An average PC will draw in the neighborhood of 200 watts. This rating assumes a normal assortment of peripherals (HDD, CD drive, floppy, CPU, 256 MB RAM, AGP video, coupla PCI cards…) and a power supply efficiency rating of 75%. It also assumes that every component is running hard simultaneously, an event that is rare to say the least. Though power needs can vary somewhat from PC to PC (Athlons need more juice than equivalent Pentiums, e.g.), I’d say you can figure each PC is drawing an average of 100-150 watts.
Most external peripherals, such as scanners, speakers, printers, etc. draw tiny amounts of current, generally from 5 to 30 watts. One exception to this statement is laser printers, which can draw 200 watts or more when they’re actively printing. (Inkjets use only about 25-30 watts.)
Finally, don’t be fooled by the rating on your speakers! Most computer speakers use a PMPO (Peak Maximum Performance Output) rating, which is meaningless in the real world. RMS (Root Means Squared) is a much more realistic measure of maximum output, which is proportional to power consumption. For example, el cheapo PC speakers typically boast a PMPO rating of at least 160 watts, but actually put out something less than a watt RMS.
Hope this helps, and happy 'puting!