rafat -
nitro's right, it'll depend primarily on what your process mix is, how big of a memory hog each program is, how many different programs you run at once, etc.
# of monitors is generally not a main memory issue since the display memory is in the graphics/video hardware and the base level memory difference between running 1 monitor and 4 monitors is essentially unnoticable.
My usual machine is a 512Mb dual processor W2K system with 3 monitors that's got more than enough space for trading and a bunch of other things, but I also have an older uniprocessor NT machine with dual monitors and only 128MB that I sometimes have to use from a different location and I can run a normal contingent of software from it.
Figure under typical conditions if you have Internet Explorer open it needs about 14-15MB and each additional instance takes about 4 MB. I'm running Trade Prospector Realtime for charting and analysis with a dozen or more chart and quote windows each with a number of studies and/or signals and it uses only about 20-25 MB, MS Outlook uses 2-3 MB, Norton uses about 5 MB, the trade entry program takes about 6-7 MB, and WNT needs about 40-50 MB. Everything runs fine on the 128 MB box.
But you've got to know your software mix and footprint requirements - if you're using some real memory hog software or a need to run a dozen different big programs at the same time, 512 MB might not be enough. But under normal conditions, I'd guess you should be fine.
What are you planning on running on the machine?