All of the x16 slots are intended for video cards... even the one wired as x4. When you get the computer, there will be a spec diagram in the manual which will show you which slots are x16... so depending upon the video cards you choose, you can use the slots appropriately. The amount of data coming through a trading rig is very, very low compared to the capability of the rig. Therefore, it makes no performance difference what video cards you use so long as you have enough ports to run your monitors. That said, the biggest player in the multi-mon business is Nvidia. They have a line of video cards, "Quadro NVS", specifically designed for use like ours. If you want to economize you can use 3, NVS 295 or 300 cards. They're cheap on eBay and are passively cooled. Unless you expand to >6 monitors, you won't be concerned with x1 cards. However, they work just fine in any of the PCIE slots with equal performance to the x16 cards. Next step up is NVS 310/315... then 420 and 510, which are quad-port cards. You can go to Nvidia's website to check out specs. Any of the NVS cards will work just fine for trading. I use 295s. Then there is AMD. They have a small portion of this market and have a new 3-port card, FirePro V4900, which you might want to check out.
Don't know. However, Dell workstations do cooling different than many others. As a general rule, Dell workstations do not have "CPU cooling fans"... which are small... to fit over the CPU... and necessarily run at high RPM (generating high noise). Dell cases are are much smarter. They use a heat-pipe, passive cooler... aligned in the air flow with a MUCH larger case fan (running at lower RPM and therefore less noise) to draw outside, ambient (cooler) air directly across the cooler. This is nothing new. They've used the same approach for more than 10 years, so their approach to "noise" is experienced and effective. I've had T3400s and currently have T3500s... both effectively inaudible from 3 feet. Reasonable to presume T3600s are similar.