Trading is not data intensive unless you are running numerous L II, or DOM screens at the same time, and doing other number crunching stuff as well.
My point is just about any decent PC works fine. I have a 4 year old Dell Dimension 4400, with an equally old quad card from Appian. Works great, and drives my 19 inch monitors at their max resolution.
You can run a single quad card, or 2 duals, or 4 singles if you have the slots for them. If you run an older PC, you will likely have PCI slots and one AGP. Buy used cards via Ebay for next to nothing in this case. Matrox G450's come to mind.
New PC? I'd get something midrange with 2 gigs RAM (and option to add more later), and 2 Nvidia Quadro NVS 280 dual output cards. If you don't do gaming, these cards are great, and cheap. Also don't have fans, thus reducing demand for a big power supply in your PC. If gaming is also needed, get Nvidia Geforce line. More expensive and consume more power though. Personally, I'd just get a separate laptop for that, but I've never played a PC game so its a moot point.
Monitors: I prefer regular monitors as opposed to widescreen for charting. They are also cheap via Tigerdirect or Newegg. $150 each or less for 19 inchers. Make sure they have narrow bezels.
An Ergotron quad monitor stand, or similar, will set you back about $200 if you shop around a bit (paid 270 for mine).
So:
Mid range PC say $600
2 Nvidia cards, via Ebay, 100 to 150
4 monitors, 600
monitor stand, 200.
Total = 1500 to 1700 maybe.
Don't buy a pre configured trading station. Will cost a fortune relatively speaking.
Most traders do massive overkill on the hardware. Even Gnome uses older stuff, and he does know his shit. Speed isn't even an issue unless you are a scalper banging away for tics all day long. A new PC wouldn't help me in the least, except for greater RAM. Even with this old machine, I can run 10 screens if I want.
Eventually, I am going to go to a 3 monitor setup with 30" screens, which will require all new gear. Expensive also, but a clean setup. Going to run current system into the ground first since it works just fine.
Jay