I google searched and it brought me to this thread, I was surprised more people aren't more knowledgeable about PC vs. Laptop specs these days. Mac Pro's are good, but very expensive for what your doing. For audio production, you would probably have to get the external interfaces anyway, but having a laptop with similar if not better specs then a PC is very doable.
SSD's are dropping in price, I have one in my desktop and wouldn't ever purchase a spinning hard drive again, put that in a laptop and you have better performance and battery life.
I've seen crazy amounts of decent ram 8+GB which is WAY more than enough, I've been running 4 GB in my desktop for 3 years now and it does plenty.
The new Ivy Bridge core i7's/i5's are beasts no matter which application, and I dont think that the desktop and mobile processors even differ since the new chips are very small. They make video/audio encoding/decoding (very very CPU intense) very easy nowadays.
Video cards aren't really necessary, just nice to have, especially if the GUI of whatever mixing program you use is semi-intense, but from what I hear about the Ivy Bridge's they have decent on board video.
Having an external sound card or interface is somewhat necessary, there's nothing great on the market that they put into laptops these days, either way you'd have to buy it for the desktop or laptop, whichever you chose. Yes having a PC is nice for upgrades, but hardware is VASTLY surpassing software, multi-core processors are getting less power hungry and more powerful and VERY reliable. If you spent around 1000-1500 on a decent laptop with above mentioned hardware, you'd be set for a while. Make sure you get one with a few USB ports for expandability... that's probably the only place I'd say desktop over laptop...even then, USB hubs are options.
Hope I didn't offend anyone, trying to help out OP.