The main factor is that notebooks with that kind of power are about twice as expensive as their counterpart desktops. I love desktops.... I am deeply sadened that I cannot continue to build them because I am moving to Japan and will not be able to ship them back and forth etc. I have pretty much built a new desktop every year for myself for the last 6 or 7 years, plus several others for other people.
Now, if spending the extra money is worth it to you (the price for portability) I will tell you that you will have plenty of power to game all night and all day at a lan party on a good notebook.
I recently took my 5720, and my brother took his dell xpsm170 to a lan party. We were the only notebooks out of the 30 people there. We also had the most powerful systems out of anyone there. The next best was a desktop with a P4 and 6800gt.
It was really nice not having to lug in all that stuff... desktop monitor speakers k-board mouse etc.......
It was a nightmare.
Also, gaming on my notebook was solid. I only have my notebook at this point and I game on it as well as everything else. I doesn't get hot or overheat or anything like that. My cpu actually sits at around 30c idle.
My point is you can game very nicely with a notebook. If you get a top of the line one you will more than likely have the best specs at most lan parties or atleast one of the highest.
(most ppl don't have fx60/x1900xtx)
My last desktop was in fact an amd fx60 coupled with an ati x1900xtx with 2g ocz platinum ram. Yes, it was much more powerful than my notebook.. but I really do not see a huge difference in the games today. Take the new tomb raider for example. The fx60 x1900xtx rig can play it in 1280x1024 resolution in next gen mode, but suffers from big slow down at times. This is also with an overclocked watercooled ati card at 800mhz core! (my friends rig) and oc'd fx60 to almost 2.9Ghz.
Now, I can play without next gen mode on but everything else on (AA max AF max) at 1920x1200 flawlessly with no slow down at all. Comparing the two, we both agreed that it looked better on the notebook with these settings.
In other words, yes it is more powerful but... you will not be running crazy stuff at high resolutions anyway.
Well, anyway goodluck in your decision. Maybe just save up and keep your desktop plus a notebook. If I didnt have to have a notebook I would deffinitely have a desktop... or two