How were you planning on selling it , E-Bay , Craigs List , or private sale. You do have to know that is is an older computer with older components. What you can try to do is list it for a higher price and then see the response and lower the price as you go. Thats what I do on E-bay and you end up selling it for close to what you want. I think if you start out at $400 and see what happens , but it mainly depends on you and what you feel it should be worth.
um... i would buy that box for abour 400 if i had the cash. it could play almost any game better than my laptop, 300$ for pentium p6100 2ghz, 3gb ddr3, integratde GPU, i wish i had gone with a desktop but i like to have a computer on the go so oh well. i got 200$ now teodo if you want that for it xD ( jk ) and nice monitor, you may as well keep that. but if you advertise it as a gaming pc yo need to keep the GPU included, maybe state its value in any advertisements you put on it, but deffinitly dont seel it seperatly. but that computer could still hold its own pretty well in gaming, especially if you upgraded GPU.
Souds to me like you may have some idea what you want to try and sell it for , the trouble is you are selling a used computer and some sites are selling new computers wit warrenties and tech support so you do have to take that into account. You can get an idea of what you can get for it by putting it up for sale at what you want to get and see wehat happens. Take the cost of the computer plus the monitor and subtract 20% and see if it sells.