I would personally look at a desktop with at least a quad core processor. You don't need anything expensive. I have a Dell Inspiron 530 with a Q8300 and 8GB memory, Vista 64bit. I've had no problems with NT 7 and it runs very fast. I found it on eBay (refurbished) when they were giving discounts, but also dealnews.com sometimes have good sales on desktops.
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I have built 4 computers in the last 18 months, all with Nvidia 9500 GT based cards. If you are building a computer for trading only, fancy graphics cards are not necessary, as you are only doing simple 2D graphics. You also don't need a ton of memory on the graphics card, but most of the cards are 512M/1G at this point.
Most cards can handle two monitors, but be sure you look for an adapter that has two dual DVI outputs, so you can connect two large monitors in the future.
Just go to NewEgg, and search on "9500 gt". There are a bunch of brands, that are all repackaged versions of the Nvidia design. I have used the Zotec brand, but it is not always in stock. There is a passive version of the Zotec, but the heat sink is pretty big if you are putting more than one in a box. Other kinds of cards work also, this is just what I have been using.
Depending on your motherboard, you can put up to 4 of these in a computer and power 8 monitors. We have built 2 & 3 adapter machines at this point and have had very good luck.
Although, I'm not an advocate for a custom-built system for trading activities, an Intel-based system should add more quality and performance to the desktop system. Also, a 128GB or 256GB solid-state drive should suffice (instead of the 1 TB HDD); the SSD don't have moving components; they are faster, more reliable, and they consume very very little power.