I am thinking of upgrading to an i7 system and was wondering if it's worth selling my old system. If so how much would you suggest?

My current spec is below:

CPU: i6700
RAM:6gb ddr2 800mhz
MOBO: Asus P5E
GCARD: BFG 880GTX
SCARD: X-FI extreme music
HDD: 150gb raptor
OPTICAL: lightscribe dvd rw and a 2nd dvd rom
MISC: Internal card reader and FDD (no idea why I bought that )

The Bulk of the pc is about 2 1/2 years old but the Motherboard and Gcard were replaced maybe 1 1/2 years ago, when the GTX was released.

Thanks in advance for any suggestions