Bluescreendeath :
1. A Q6600 is equal to or marginally better than a PII when at the same speeds. A Q6600 can overclock to 3.6GHz easily and 4.0GHz and over with some effort.
2. Ebay usually have Q6600s for $120-$140. There have been people here on tomshardware's classifieds selling it around the same price as well.
3. He already has a 775 motherboard so the best & cheapest option is just get a Core2Quad. Yeh buying a new AMD PII and AMD motherboard will give him more performance, but the performance wouldn't be worth the time and cost.
1) Correct. Intel's Kenstfield is roughly equal, to just a hair better, than the Phenom II arch clock-for-clock.
I think you are confusing the overclocking ability of the 45nm Yorkfield arch with this 65nm Kentsfield though.
While a Q6600 can be pushed to 3.6Ghz, it is quite a stretch and requires very good cooling.
I have never seen one running above 3.6Ghz with out excessive and unsafe voltage.
A more realistic goal would be 3-3.4Ghz, assuming the cooling is good enough.
2) Sure you can but how bad has it been abused in the past?
I personally would never buy a CPU, or any used computer component for that matter, from fleBay.
3) Cheapest, yes.
Best, debatable.
Keep in mind that the nForce 680i is a heat machine and a questionable overclocker.
I would not expect to get more than 3-3.2Ghz out of a used and abused CPU and this board.
From what I remember of this chipset, even that may be pushing it.
Yes, getting a used Q6600 is defiantly the least expensive upgrade path.
Personally, I would consider that $100ish better saved towards a $300ish platform/CPU/RAM upgrade.
My 0,02€