In my post Day Trading Lesson: The Roadmap I present to list of steps to learn to Day Trade.  Here are the details of Step 7:

Get a good trading computer and Internet connection

As a Day Trader you depend on your computer to execute trades and analyze charts.  So you need a computer that can do these things quickly and reliably.  And you need a high speed Internet connection that doesn't often go down.

Meet these requirements and you will be OK.  A crappy computer or Internet connect will cause you aggravation and even cause you to lose money trading.  On the other hand, past a certain point, spending more money on a computer will not make you a better trader.

This post will not give you all the info you need to choose a good trading PC, but here are few guidelines.

Get a decent processor.  Look for at least dual core, at least 2 GHz.

Get enough RAM.  Get at least 2 GB.  Many decent PC's come with 1 GB standard, my advice is to shell out the extra bucks to upgrade to 2 GB. 

Hard drive space is less essential than RAM, but given it's cheap look to get 500 GB or more.

Most traders like a lot of monitor space.  I would recommend at least two 19" monitors, three is even better.  Be aware that 3 monitors adds quite a bit of setup complexity and expense.  You can't just buy your standard off the shelf PC and attach 3 monitors to it. But IMHO 3 monitors is really the way to go.

If you are purchasing a PC and you're not a "techie", it's a good idea to purchase from a company that will help you choose the right PC and get is built and configured for you.  Call up a few retailers and speak to their tech support people about your purchase requirements.  Remember good support can be really valuable.