With LGA 775 these are as good as it gets, Core 2 Quad QX9775, QX9770, QX9650,Q9650,Q9550, Q9450, Q9400. The QX9XXX chips are hard to find and Expensive plus not many motherboards will run them "high End boards only" but overclocked they are neck and neck with chips like a FX-6300. You can find a QX9650 on Ebay at the moment for just under $50 but that is rare and wont last long.
Q9650 to Q9400 overclocked are on par in gaming with FPS you will get out of most current I3's which isn't bad for a older chip but budget boards generally don't except them.
Q9300, Q8400, Q8300 - A Step down from the above, less common and will work on most moderately priced motherboards.
Q6700, Q6600, E8600, E8500, E8400, E7600 Are more widely accepted and overclocked will still run Low to Medium settings in nearly all titles.
I still build quite a few LGA775 systems as parts are dirt cheap and common they still have life left for budget gamers saving for a better system. What is your motherboard btw? 50% of all LGA775 systems sold top out on a Q6700.
A Core 2 duo "E" is a Dual core, Core 2 Quad "Q, QX" are obvious. Should you upgrade everything, CPU/Motherboard/RAM? that depends on what you expect, if your wanting 1080p Max settings then upgrade. If you can deal with lower settings or even Lower resolution with higher settings then look at upping your CPU, this all depends on what you are willing to spend.