Things I should consider with my first homebuilt system?

I would recommend that you ask here and let use know what the purpose of this computer is for...workstation or gaming or so on. If you look at the motherboard you are buying it will let you know what parts will work for it. Best bet is to buy all your parts together to test them out. In the case that a piece of hardware is bad and you may RMA asap.

+1 for paq. I bought the pieces for my current pc over a year+ period and did have some rma issues. Ended up buying a different PSU and an Antec 900 case etc.

However I am a college student and no way possible I could afford to buy everything at once.

Important factors to consider:
What will you use this system for?
Will you need to include monitor, keyboard mouse, shipping, etc in final cost?
If gaming is involved plan on spending a good portion on your video card, or you'll regret it later.
Do you want hi def?

+1 - just give us an idea on purpose and budget. Quite often, we can build far cheaper here.

We're more than willing to help.

for multiple monitors, almost any add in graphics card (from either ati or nvidia) can give you at least 2 outputs

adding in another gfx card usually works without a hitch if it is same make and generation, most decent mobos have two pcie x16 slots so you can add in a 2nd card (usually this is for crossfire or SLI setups but it can be used to drive more monitors as well)

if you are doing securities trading and running a bunch of apps at the same time and presumably on different screens, a quad core 2 or even a new core i7 based system would be the best bet

I'm with everyone else here, take a close look at your needs first but by the sounds of it (trading and gaming), you need a really fast rig based on the i7 920 processor & corresponding platform. You would also want to consider very carefully your future upgrade path in light of your needs when you make such a substantial investment in a system.

Look carefully at motherboard design when considering upgradability (expansion slot design, features).

It's no problem to go from 1 monitor to 2 as any video card that anyone here would recommend to you can run an additional monitor without additional changes. You'll probably go with a dual GPU setup--for gaming, not the 2 monitors.

You would want to consider cooling vs. quietness for the case (as well as appearance), as well as a full tower for maximum airflow and expandability.

Consider how you want to setup your network adapter (wireless or onboard), and maybe the configuration of the rest of your home network too.

Think about upgrading to a RAID 1 array for data backup and security later on.

Since you're trading, you'd want to select components for quality and stability over time.

I would build the platform first, with as much of an open-ended upgrade path as possible on as many factors as possible, for maximum future flexibility.

I know I'm jumping the gun here but this is what came to mind when I first read your post. Do an "alpha test" when building (i.e. everything except drives and expansion cards) to cover defective components before and after the alpha test.

You've got a sophisticated build going and we're all more than happy to pitch in for you.

Good luck!

OH, one more thing. I'm aware that for a sophisticated high end system I might be better off having it done by a professional. However, having tinkered in small ways with my PCs for several years, I've really determined that I want the experience of building my own PC as well as the knowledge gained in that endeavor.

meh building a computer is easy it's like lego, only with more expensive pieces lol
as long as everything is compatible then you are good to go

Regarding cooling vs. quiet, that is a generalization and it is also a matter of degree. One way to increase airflow and cooling is to use a mesh enclosed case with lots of fans inside. That means that you can hear all the noise inside your case, especially those GPU fans. But there are ways around this, like turning down the fans with a fan controller or using a larger quantity of fans to move the same volume of air. On the other side of the coin, making a case quiet requires some form of sound damping, which tends to trap more heat than mesh. Hence, the general tradeoff.

You're right in that a "professional" build comes with tech phone support if something goes wrong. I'd agree with you, though, on the second half of that thought--once you build, you will have an idea on how to troubleshoot. Plus there are plenty of "professionals" here on these forums to help you out as well. However, if you're trading volatility positions by way of collars, then it's a risk issue for you and you might want to reconsider...