1. RAM is probably the most important, and in ANY modern machine the capability of its RAM is waaayyyy overkill vs. what's necessary for trading. 2. Though you have concluded, "it's worth it to pay a few hundred bucks"... it likely is not... even for your "real money on the line"...
Again I disagree, CPU performance is most important. That's your opinion...perhaps you should have made your luddite views known to the OP before you started offering your advice on his system upgrade.
Are you sure this was due to a hardware problem and not a software problem? New hardware may be an expensive solution to a problem that could be solved by simply reinstalling your operating system (I'm assuming you're running some version of Windows). Even if it is a hardware problem, it could be a very inexpensive fix: a new stick of RAM, a new HDD, etc.
How could it be most important if it sits most of the day at idle? I never said there is anything wrong with i7. It's just (1) a gamer machine CPU, (2) runs at 135W vs 65-95W's of other modern, fast CPUs and (3) is more overkill, and (4) is therefore not the best (depending upon one defines it) for trading. Not letting the facts influence their opinions, people tend to defend the choices they've made.
XPS is OK. Has x58 chipset. Some have only 1 PCIEx16 slot and no PCI. (For 3-4 monitors, 2, x16 are better.) Dell's mobos are mostly Foxconn OEM. PSUs OEM also. All other parts are "off the shelf" like everybody else.
The speed at which your computer responds is determined almost exclusively by the CPU (assuming the configuration isnt screwed up). Just because you don't run a CPU at 70% all day long doesn't mean its not worth it to have a fast processor. When the cpu is given a task you want it to execute as quickly as possible. Again, it doesn't matter if it executes quickly and then goes back to a lower utilization state (which you incorrectly categorize as idle). This is really all very basic stuff and not worth arguing about - believing that only "gamers" need fast machines is sad.