Well, here's what I use for a 4-monitor rig running multiple apps like everyone else... + a TV tuner to occasionally watch CNBS (mostly for watching golf, bowling, billiards though) Dell Dimension 8300, P4, 2.8Ghz, 1GB RAM, Nvidia Quadro NVS 280 AGP + 280 PCI video cards, WinXP Pro. This is plenty of machine for most users. Nothing special for hard drives... just adequate space and some backup capability. You don't NEED RAID, as there are other alternatives. You may have higher-than-average horsepower requirements depending upon your software, but you should know that before you buy a new machine.
Depends what you trade. I now a guy who successfully trades ES with a measly laptop running celeron M 370 with 2 gig of Ram. This thing does not even have a dedicated graphic card. Do not fool yourself into thinking that you need a $3K rig to get paid.
I just built my own machine. My advice would be to go local. Find the best computer shop nearby, tell them what you want, and the price will be lower then a dell 7 times out of ten. This is what I just got: E6600 Asus P5W deluxe Crucial Ballistix PC2-6400 CL4 (4* 1g) Seagate barracuda 320g Sata2 hdd Geforce 8500gt video Antec Sonata3 case w/ 500w PS Vista 64bit business Add another 320g HDD(the board supports RAID) and a monitor and the price will be lower then the two you have shown. You may also want to get a better video card like a 2D Quadro I would also go with a 64 bit operatin system if your software will work with everything.(look on the product's forums) I just tested mine out with 136 charts running and the CPU load was 22-30% using a 32bit charting program. AAA30
Just get the fastest machine possible without spending a huge fortune. If you work with a computer for 8+ hours a day, you start to notice every small delay no matter how small. After a while it becomes annoying and slows you down.