There are 2 basic PDA's on the market (well actually 3 but more later); Palm based and MS CE based.
Both have tons of free and shareware programs for tracking expenses, mileage, billable hours, note/letter writing, spread sheets etcetera.
Palm tends to be a more stable OS and does have 'instant start' capabilities. Windows CE is a scaled down version of Windows with limitied functionality and 'fast start' capabilities. Both units will synchronize with you PC based computer and the Palm supports Apple computers as well.
Palm computers are as they are named, small enough to fit in your palm. They have some sort of hand writing recognition program to allow for taking notes and inputing information. The Palm OS tends to be more compact code than the MS OS and is >50% market share. MS has ~45% with the remainder being the Psion product. Psion makes a true hand held computer that is fast, reliable and stable but limited availability.
Prices range from $100 to $1000 depending on memory, screen size, colour screen, lighting et al.
Note book computers are smaller in size, feature and functionality than lap top computers. Note books would be sized like the iBook or Sony Vaio; larger units are considered lap tops.
As has been said any computer is basically a GIGO unit. Garbage In, Garbage Out. Data is data but you need to use it to benefit.
Start with a paper based system like Day-Timers, Day Runner or any others. Once you get used to doing the grunt work then you can find financial and time justification to invest is an electronic device and begin to let the computer analyze your data.
But.....remember what Confucious said, "The palest ink is keener than the most retentive mind."
I use pen/pencil and paper for any note I absolutely must have.
Hope this helps? Cheers.