I can not commend on prices or quality of the computer brands discussed above. I suppose you are going to trade from home, so I recommend that you have a look at the noise generated by the computer you are going to buy.
Fast CPUs, high-powered grafic cards demand a lot of cooling, the cooling fans used differ widely as to the noise they make.
Fast revolving hard-disks (7200 and above) make a lot of noise. To dampen this you need some kind of floating construction to fix them inside the tower and some noise management. Which essentially results in defying the fast rpm.
Ask yourself whether you really need such a high-powered pro system. The noise can become irritating.
Regards
Bernd Kuerbs