I recently bought a new home computer - I wanted to upgrade along two dimensions, and I probably could have done so with my current machine, but ended up just springing for a new one. Those two dimensions are:
RAM - went from a mediocre 512MB to a well above average 4GB)
Monitor Size - went from a cheaper 19" LCD to a dell ultra-sharp 24" widescreen LCD. My graphics card can support 2 of these 24" screens for upgrading.
The excess RAM is mostly so that I can experiment with writing code that actually makes good use of RAM - not a reason most people are concerned with.
The monitor however is something I would recommend for everyone who does any significant amount of work on the computer, even if that work consists of something as simple as writing word documents.
Granted, it is pricey. The 24" monitor was around $650 new - but dell has refurbs available for about $100 less and the prices are coming down. They were $750ish last year.
Still, the productivity gain is absolutely amazing. Google provides it's engineers with dual 24" monitors standard, and the newest thing is to offer single 30" monitors. There have been a few published studies from Microsoft and some other companies that indicate that increasing screen size in controlled experiments improves productivity by 10-40%. The cost is inexpensive given that.
Not that long ago, I helped to set up an informal study (the experimental design wasn't as rigorous as you would need to publish the results) like this at Google for some folks doing their *real* job, instead of a controlled tasks experiment. The results showed similar productivity gains, on the order of 20% on average.
Yesterday, Republican Senator John McCain visited Google for a "fireside chat". A little surprised to learn that a Republican would be visiting Google, I went along to the show. The first question asked of the president-to-be was:
Given that this event is basically a job interview, How do you determine good ways of sorting 1 million 32-bit integers in two megabytes of RAM?