I only run notebooks, both work and home, both are Dell Latitude D630s. One XP (2 GB memory), one Vista Ultimate (4 GB memory), both 32 bit. Neither is having any issues with LR 2.x.
Purchase the fastest processor you can afford, it is expensive to replace.
Memory and disk space can always be added or replaced fairly inexpensively. But plan for this, such as leaving a memory slot empty.
I would suggest a 7,200 RPM drive in the notebook for performance.
Bob Duke