The Intel Core 2 Quad 9650 (3.0Ghz), it's what I'm running, just make sure to have the copper heatsink & not the short low cost aluminum. Here's the one I purchased for $14.99 shipped, although different seller. May need to dust out, unlike HDD's, these should be good. Make sure to clean the contact surface really good with 91% alcohol, using coffee filters, these doesn't leave lint. Clean until no more thermal paste, which normally shows as a grey/silver color, can be seen on the filter. Same for the CPU, although be very careful, handle gently. May need to start out with a Q-Tip on the edges to remove thermal paste that's dripped down from applying too much. Like with the CPU cooler, clean until coffee filter doesn't get dirty, then make sure both are dry, and use your favorite thermal paste (a oat sized drop in the center of CPU), ease down cooler & give a very slight wiggle, tighten down, although rotate, using 3-4 rounds on each side to tighten down heatsink.
https://www.ebay.com/itm/OEM-Dell-J9761-CPU-Heatsink-Mounting-Bracket-for-OptiPlex-620-745-755-760-MT-/122465131396
Here's my Speccy specs to show my Optiplex 780 MT, although my smaller (mid-sized) DT edition runs the same CPU, both maxes out at 16GB DDR3 RAM, although am running 12GB, had to balance what I had between the two. Didn't want to spend the $120-130 for two more 8GB kits. Plus there was absolute zero increase of the WEI score (to my huge disappointment) when going from 8GB to 16GB on the DT edition.
http://speccy.piriform.com/results/TXom1FuFB0KTJCNtwcQVnry
Q9650 is as good as it gets, while there's a faw 130W variants of the Core2Quad, I believe 95W is the limit as far as Dell motherboards goes, even if the CPU physically fits.
Also, note that you can save nearly 50% by going with a Q9550, has the same 12MB L2 cache, just a slight difference in speed (2.83GHz) & again, if the WEI drops from the normal 7.3, it'll be just by one-tenths of a percentage point (7.2). Many jumped on the Q9650 at launch, which led to highly jacked up pricing (near $600 at it's peak), when the Q9550 remained stable at around $250-275. I have both in different PCX's and to be honest, can't tell the difference one way or the other.
That 12MB L2 cache means a lot more than a 6MB L2 cache with 4MB L3 (if that). L3 didn't mean a lot with these CPU's, why many didn't have it until the 1st gen i7's came along, and later, some of AMD's offerings.
So to make a long story short, the Q9650 is the best that Dell approves for the MB, and was a true legendary CPU for it's time, and not too shabby today, am sure it still ranks high on the list of Intel's most popular CPU's. If you could luck up and find a brand new, factory sealed box today, would be worth every cent of $500 to many. On eBay, there's lots of one owner models still asking for $75-100, although pricing is falling. Plus sellers are competing against the Q9550, will repeat, cannot tell the difference in performance.
Good Luck!
Cat
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