E6420 does have the extra cache, you are thinking of the E6400.. and the extra cache is only 2-3% in real world benchmarks anyway
All the E6420's will run at 3.2GHz on stock voltage, get yourself some nice DDR2 6400 (800MHz) set it to 2 to 1 in the bios and go for 8 x 400 on...
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