People may reasonably marvel why they should pay much extra for having an 60GB rather than a 40GB hard drive, or a 2GHz processor rather than 1.8GHz. On the face of it, the price may seem designed for people who want the best, and the abstaining consumer may be proud to be a member of the less easily fooled. However, do consider that your operating system may consume some 10GB of hard disk space, so now you get 50GB rather than 30GB. Sounds better already? The same goes for RAM (memory); it is anticipated that the next Windows version may consume as much as 512MB of memory just to run. So if you bought 2GB total memory rather than 1GB (as a hypothetical future example, these configurations would currently be thought unusually performant *cough*), you'd have 1.5GB free vs. 512MB. Quite a difference, non? You've suddenly gone from factor 2 to factor 3.
Thinking about CPU speed is slightly more complex, because the benefit depends whether your system usage scales with the clockspeed or not. If you have lots of CPU intensive services running at regular intervals (such as a web server, not unheard of!), you will benefit from having those extra 200MHz extra, although whether you realise this, and are actually more efficient because of it, is up to you to assess. However, personally, choosing between a 1.83GHz and a 2.16GHz CPU, I would always go for the lower end. For many years, chip manufacturers like Intel have worked hard to make us believe that your computer's performance depends crucially on the clockspeed. Experts know that RAM is more important, and stuff their machines full of it. (This is partly the fault of PC sellers, who will invariably package the smallest amount of RAM that will keep the machine going. Currently, this would be 512MB. Even 768MB will give you markedly better performance, and may cost you less than a 100 GBP to upgrade!)