I upgraded from a 32bit Core 2 Duo to a 64bit i7-2500 about 1 year ago.  My new PC is pretty minimal for Reality.  I get reasonable results in I think reasonable time.

 

RAM will cap the size of render you can run.  The biggest I've run is 12GB.

My PC is an i7-2500 3.2 Ghz with 16GB of fast fast ram.  I use a SSD for my main PC drive, and a super fast secondary drive for my library and games.

Initially I had a stock fan in the PC, but it started to sound like a weazing old man when a render started, and I tend to run them over night.  It was loud and kept waking me up.  I upgraded to a water cooler in preparation for one render, and overall the PC is louder but quieter when I'm rendering.

http://www.renderosity.com/mod/gallery/index.php?image_id=2417185&user_id=707942

The other upgrade was a NVidia Ti560 with 2GB of RAM.  Its been a champ for games, and you need more ram if you want to try video card rendering.  (Check out Octane.)