May be hard to sell as a whole, since most gamers tend to build their own PCs with their own components.
Any other casual PC user will go to Dell and get a cheap sub £300 machine for their needs!
When I build my gaming monster a year or so ago, I made sure I kept all the component boxes. I ha dthe PC for about 8 montsh or so and was bored of it. Took everything apart and back into original boxes, and stuck the lot on eBay. Pretty much got what I paid for the PC in the first place!!
Im guessing you dont have boxes or anything though, or dont want the hassle of taking apart and selling seperate?
Personally I would sell the monitors and the peripherals seperate. Then sell the base unit alone.
Off the top of my head, without looking at any component prices, I'd say you may get around £500-550 for the lot (on a good sale) if the monitors and peripherals are sold seperate to the base unit.