You need to look at what goodies you want like do you want anti aliasing, a good resolution, anisotropic filtering, pixel shader.  Also your budget is a major factor in what you can choose from.  At the top of the range there are the Radeon 9700pro for laptops, then they average out at 9200np. 
I am unsure about what Nvidia cards are available for laptops but i am sure someone will post here what are.  I would expect atleast the FX5200 would be available for laptops.
You will need atleast 32MB of Graphics RAM to run this sim decently, i would recommend atleast 64MB though.  Also avoid laptops with onboard graphics cards, they waste system resources and offer no benefit.

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I just re-read what he said and realised it was desktop not laptop

Yes i second the FX5200 if you are on a budget, look at the 9600np if you can afford a little more, because it will allow you to max out the water setting.  If you want more settings to be maxed out look at something no less than the 9600pro or 5700U.  If you choose the FX5200 check that it is a 128-bit version first, the 64-bit versions are a lot slower.  Also for ATi cards generally avoid any that have "se" on the end of them.

Just wait before you get that new PC. Graphics cards are about to undergo a huge price drop with the next generations being released. In fact nVidia's new Geforce 6800 was released today.

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The FX5200 actually has a 256-bit core. Remeber that FS2004 likes video card memory. I'd recomend at least a 128mb DDR model but a 256mb DDR model is better. I highly recomend the FX5200 for a low end card. I've had it for almost a year now and I've never had a problem.

As for CPUs, I like AMD. They're good for gaming. Don't be fooled by their clock speed. AthlonXP's are cheap right now but a look at the Athlon64 may be worth it as well.

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All graphics cards have 256-bit core.  I was referring to memory bandwidth which the fx5200 has 64-bit or 128-bit. 128-bit being the fastest of the two.

Also no games use 256MB of memory right now.  Doom 3 which is the most demanding game to be realised very soon, will use 96MB, so FS2004 is not going to use anything near that.  So buying a 256MB card now will not be a good idea, because by the time programs come out that use 256MB the graphics card will be to weak to run the program.  Also 128MB models are often clocked higher and/or have a better memory time.

The 6800 is rumored to have fixed all the problems with the FX series. It's supposed to be twice as powerful as an FX5950. It has 400mhz memory speed and a 1.1ghz core clock.

ATI is also developing a new card. I personally prefer nVidia but they're worth a look at.