One reason the laptops may seem smoother is the resolution. What are you running your laptop at compared to desktops?
My laptop is 1440x900 (I could've upgraded it to an even higher res, but this was already an upgrade, I think 1200x800 was stock?), but my desktop is 1280x1024. Now in my case, we're talking about only a mere 15000 pixel difference, but some people have widescreen monitors running at 1680 x 1050 (which is 450,000 pixels more), 1920 x 1080 (700,000 more), or 1920 x 1200 (1,000,000 more).
Since ED's sims are very CPU-bound, higher res needs more CPU power. The only thing a bigger video card gives you is the ability to crank up the FSAA.