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From the Core2 to the Core i-series, Intel integrated the memory controller in the CPU. That cut memory latency roughly in half along with a ~60% step in IPC. The Core2 is simply a non-starter for most modern games.
As the TechSpot benchmarks show, almost anything current-gen you can buy today will be more than twice as fast as a Core 2 Q6600.
OK, Recap. most 775 boards can take an unmodified* E3 or E5 series Xeon; although the 3.1, 3.2, and 3.3GHz Xeons are a priced in the $40-$60 range, the 3GHz E5 5450 is about $15.
The Q6600 mentioned above is a 2.4GHz chip, so a fair bit slower than the 3GHz E5 5450.
A benchtest of the E5 5450 in an ASRock mobo with DDR2 RAM gave similar results to an AMD FX8xxx series cpu with DDR3 RAM, allowing for the fact the FX cpu has twice as many cores.
Any later generation gfx card wont be able to run at full speed, due to the PCIE gen1 slot, but the loss isnt huge, perhaps 10-20% in the most intensive games, less in older titles suitable for the age of the system, as long as you accept you arent going to be gaming in 2K or 4K.
The biggest limitation is if the board only runs DDR2, the max these boards can take is 8GB, but 4GB modules are stupid money, similar in price to DDR4 16GB modules, so realistically, unless you are very lucky, you are stuck with 4GB.
BTW, if anyone has been running a DDR2 board, is retiring it and HAS any 4GB DDR2 modules, could I have the ram please?? (For my 5 y/o's PC).
* The mod is just filing a small notch in the pcb the cpu is mounted on; if you buy from Ali Express, most of the Xeons will be ready to drop into a 775 socket with no further mods needed.
PS, those FM/FM2 cpus are utter crap, after upgrading and finding most of my games ran slower, even when extremely over-clocked, I went back to a Phenom II 1090.