I would buy a "cheaper" desktop at home and a more expensive desktop at the office to render off of. Or vice versa if you want lol! $6K can get you a sick amount of performance today. Set up a way to access all of your files in the office from you home computer via log in and shared folder, or by the old school archive file in max and a portable HD as mentioned before. The advice you have gotten IMO is very sound (unless someone is trying to sell you a machine.)
I would just forget the laptop. It will be heavy to carry around, it may run hot, and if you are running it on a battery while rendering, it will die quickly. Just not a good idea IMO. Also as a laptop heats up it may run slower. The only thing i think would be useful on the laptop is to tweak the files, send em in and render that way. Dont bother rendering on one. The screen on a laptop will likely never be close to as good high end consumer monitor as well. You can have 2 good 24" monitors as mentioned before.
The other thing to think of is that there will soon be dual Nahalem chips on a workstation in PC, and the price will be very low for the power. Apple has a deal with Intel where they get great prices for their chips, and get them before anybody else for their macs. There are already dual Nahalem Xeons in the power macs on apple. It hasnt been released for PC yet, but they will be out soon. If you cant wait, which i assume u cant........
You can always see prices of parts on a site like pricewatch or zipzoomfly. to get an idea of what someone like Dell or Boxx is charging and what the parts cost and how much people are making off of their systems. You can tweak your graphics card or ram very easily by getting the minimum even from apple for a power mac and putting in a real graphics card and a bunch or ram. Dont believe the fear tactics that things wont work and it will be so hard, etc. When the last Xeons came out, the best buy was a power mac. If you tweaked the graphics card and ram, and run boot camp. I rendered with it. Incredibly stable. It was simply cheaper than any PC. I assume their agreement w/Intel will make this the same for the near future as well.
What i would do is the following if you can get two machines and the best bang for your buck IMO. Forget the "pro" graphics cards, they are just a way for Boxx and Dell to make more $ and they are so powerful now, that truthfully these apps cant use all of the power, only a high end video game can. They work just as well. I have had problems w/high-end cards for 3d for what it is worth as well.
For rendering and your #1 machine get a mac pro minimum specs except for ram, get 8 gig for extra $100. Upgrade on your own to a radeon 4870 1 gb graphics card at $200 and buy another 2 pieces of 2bg ram to get u to 12 gb, or if you want to go wild go higher w/ram. Get one more HD and install your windows OS on it. Get one nice monitor. Maybe a samsung 24" tft? If you like Apple monitors, samsung makes them and the right model is better for color and other things. Run boot camp and XP 64 or Vista 64. I assume you maybe a bit over $4.5k. Next machine is a beater, but now you can get a very fast machine for like $500. The monitor is up to you. Likely this machine will be good enough for what you want to do anyways. Believe me, i use my beater all the time, and its not chopped liver if you know what you are doing.
That is what i would do, and w/the performance you will get w/that mac now, simply unbeatable.