Getting back to the OP . . . just finished the
specs for a PC I'll be building for my old man, interweb browsing, on-line poker and light MS Office stuff. Advantage of a DIY build (for me anyway) is that I know that, given the budget and requirements, it's exactly what is required. For sure you could buy one pre-built with the same specs a fair bit cheaper, but NOT with those exact parts (ie you'd be getting a cheap no-name power supply, a crappier case etc).
Point is, slap any £150+ graphics card in there to get the 3 monitor solution you want and, unless you're doing HFT, I reckon it'll serve fine as a trading/multi media PC.
As an FYI,
this build +
one of these gfx cards is my current trading/gaming (everything turned up to 11) rig outputting to the twin Dell2711 monitors (so giving me 2 x 2560 x 1440 pixels) I mentioned earlier