Reason for speed/price difference.
The speed of the chip is calculated by speed of the front side bus x a fixed multiply factor of the chip.
FSB is quad pumped so always quoted as 4x faster than reality.
So for a E8400 which is 3Ghz and 1333Mhz fsb (actually 333.33MHz) it would be 333.33 x 9 multiply factor = 2999.97MHz or 3Ghz
The cheaper chips work on an 800fsb just like your current P4 does.
The size of cache also makes a difference to he price.
Larger L2 cache is great for games but not sure what benefits normal apps.
My folks have Vista on an E2200 bsed machine and it’s not the best to be honest even with 2G of RAM.