What is the best Core 2 Quad?

I'm getting an old Dell Optiplex 780 with a Core 2 Duo from my friend for free, and thought that it could be a much better main PC than my 2014 HP Laptop (With a broken DVD drive and keyboard lol) even though this laptop has decent specs (for me anyway). So I'm planning to upgrade that Optiplex with 8 Gigs of RAM and with a Nvidia Geforce GT 730, but im not sure which Core 2 Quad model i should choose for upgrading this. Any suggestions?

 

Something from the e8 series.
e8600 maybe? Idk much about them.

 

Not all Core 2 Duo systems will accept a Core 2 Quad.

 

I know, but i checked that this Optiplex's chipset does support Core 2 Quad (Intel Q45 chipset)

 

I think a Q9650 going from this...
https://www.dell.com/downloads/global/products/optix/en/optiplex_780_tech_guidebook_en.pdf

Have never own that old of a model optiplex so I don't know for sure...

Also... you didn't mention which OPTIPLEX 780...

Like the smaller OPTIPLEX 780 USFF may only be able to handle up to a E8600 due to wattage limitation.

 

im pretty sure that those old optiplexes dont support the Core 2 quad Extreme series. i could be wrong though.
My advice would be to throw a good old q6600 in it