This is what I know...
Processors:
i3 is for the average user. Cheap and value for money.
i5 performs faster than the i3 and forms the mid segment.
i7 is for gamers, high CPU usage, video rendering, etc. Expensive.
Hard drive:
When people buy Hard drives, people overlook the 'RPM' part and concentrate on the disk space only. Make sure you get a 7200 RPM (Rounds per Minute) HDD. More RPM = More speed.
RAM:
2 GB should suffice but you could consider a 4 GB one if you want your 64-bit system's full potential. More than 4 GB RAM require i7 (As far as I know)
Graphic Card:
256 MB should do it, but at today's development get yourself a 512 MB at least.
Cache:
http://wiki.answers.com/Q/Explain_how_L1_cache_L2_cache_L3_cache_differ