You wont see much difference at all.
Personally I'd sell the core2quad (and ram) on an auction site and use the money to buy something like a g860 with a modern (but cheap) motherboard - that would allow me to upgrade it in a year or two. Socket 775 is pretty dead now.
I am about to sell my PC's components and upgrade my system also
Currently you can get a reasonable price for the Q6600
Here in the UK on ebay they sell for about £45 (USD $70)
The 2x1g Nvidia SLI 1066mhz (I have 2 sets of this RAM to sell also) is worth about £20 (USD $31)
should go a fair way towards a system with about twice the processing power - and also will save on your electricity bills too - one of the main reasons I want to upgrade as will save me £9.50 (USD $14.84) a month
I think we pay a lot more for electricity in the UK the our US neighbours
last time I checked my bills was about £0.13 ($0.20) per KW/hour
and I should get a lot more folding@home points as a small bonus