My advice on trading machines, is spend as little money as possible to get the best bang for the buck you can. If you know how to put it together yourself, you can get a 3 or 4gb barebones system for between $150 - $200, and you can add a 1tb hard drive for less that $100 and a cd-rom for between $12 - $25. What else could you possibly need?
The problem with paying a lot of money for a machine right now, is that in a few years a whole bunch of new things will be the new standard. USB 3.0 is supposed to be available sometime in 2010, later this summer solid state hard drives are going to finally start coming down to a reasonable price point, and we're on the verge on transitioning from a 32bit world to a 64bit world, as Microsoft has already stated that Windows 7 is going to be their last 32bit OS.
Back in 1998, when the average computer sucked (as did broadband), I think it made a lot of sense to pay big bucks for a trading machine and there were definitely some convincing arguments for leasing vs. buying since you can deduct 100% of your lease payments (assuming its a business lease) as an expense vs. having to depreciate the value of it over time on your taxes (although I think the IRS has a rule that lets you take a 1 time write off up front, but I'm not a tax expert). But nowadays, computers are so powerful and so cheap that I think paying big bucks is a bit of a sucker's game.
And as a side note, I know a few guys who work for some big names in the tech industry. A few weeks ago, I was thinking of adding 16gb of RAM to my machine so I could get a little ahead of the technology curve. So I asked a few of them, how much memory do you think will be "standard" in 5 to 10 years? Most of them said 64gb to 128gb. Their position is that we're moving (rather rapidly) towards a point where the average computer will have as much RAM as the first generation of SSD's.
Now they may be right or wrong, I have no idea, but I doubt that whatever you buy today, no matter how much you pay for it, is going to last you that long.
Just my 2 cents.