Quote from indahook:
. . .I have been trading in an office for 2 years . . .
You've got a baseline on the system you used in the office, so start from there. If you are a pro (ie supporting yourself and your family by trading) then you do not need one broadband - you need two. You can buy a router to hook up your LAN (that's geekspeak for the network inside your house) to a DSL and a cablemodem.
Be sure to put your network gear on a UPS as well. If the power drops and only your PC is running, all you can do is play games.
Most folks recommend a backup PC. I've got one, but I've never had a problem with my main.
More RAM is almost always better than more or faster disk. You'll need 256 meg, and I encourage you to get a gig.
Get a Lian-Li case. You'll never go back to cheap Anatechs again.
Consider using Matrox Dual G450s for multi-mon. Cheap and good enough for charts.
Check out reviews at places like SharkyExtreme, Anandtech and others. Get the best prices on hardware at pricewatch. Look for sweet deals on hardware in the hot deals forum at Anandtech
The fastest CPU always costs a lot more than the one behind it. Choose your price point carefully. It's unlikely that you will want the fastest or the next-to-fastest. Design your system so that you can get at least one CPU upgrade before you have to swap out other components.
Run XP or Windows2k.
Study the info on the different chipsets, and particularly benchmarks. I only do this when I am designing a new system or an upgrade - been a while so I'm not current and cannot give you any advice on which is best.
That's about all I have off of the top of my head. Good luck! Building your own system is very satisfying and you can design a better system that costs less money than the name brands.