Hi
I recommend the following for bang for the buck value:
Desktop: Intel Quad core (Q6600 is great value but there are other 9600’s whatever - that are really good too) cpu with 8 gig ram on a MB that supports raid. Midrange graphics card and a couple of big fast SATA disks as raid and a third disk for the operating system and programmes.
I recommend windows XP 64 bit and later maybe the new Windows 7 when it arrives and is stable if that is at all possible - hehe.
Whatever you do stay clear of VISTA - it is the single most stupid and downright unfriendly operating system ever invented.
Mobile notebook whatever: Again a Quad core or dual core with 8 Gigs ram and as big and fast a disk as possible.
OK Having said that please understand that there will be a lower performance on the notebook no matter what you do. That is just the price of mobility.
However you can still perform all but the really big polycount stuff and heavy rendering on a mobile.
I suggest you get a small desktop case that fits the stuff and put a handle on it for mobility. I have had one like that for 4 years and it served me very well. I know it’s a bit cumbersome but if you are really needing your power on a job outside of your studio or whatever it is really great.
Now I just use a notebook for this aspect but I hardly ever leave my studio to work these days.
Please understand that there are far better solutions and more powerful ones, but you then MUST spend a lot more money.
I just advised on a bang for the buck scenario.
PS I would not go for macintosh solution for allsorts of reasons that I will not mention because there are a lot of very sensitive mac users that will flame me if I say anything - hehe.