Sandy Bridge is pretty dated at this point. Two issues. It's going to be hard to find a motherboard. Memory on Intel was DDR3 up until after Devil's Canyon. I'd say you should look for a motherboard/CPU combo and upgrade your memory.
What's your budget for this upgrade?
Did your motherboard break somehow or do you just want something better? Because that CPU is from 2011. Every few years there is a new socket that the newer CPU go into. If you are wanting something new, you need to get all three major parts; CPU, motherboard, and RAM. It will be impossible to fit that new into $300. If your motherboard is just broken and you want a replacement, look on ebay for one with socket 1155.
Since you do have a dedicated graphic card, I'd advise you to go with a i3 10100f and a b560 motherboard, that would cost somewhere around $200 as the 10100f is currently on sale for $70 at microcenter. the i7 2600 is really outdated, and older i7 sometimes gets beaten by newer i3. And if you want to max up the budget and get the best performance, I would advise to go with amd with a $200 5600g + a budget b450 motherboard. For processing power comparation, refer to this: https://cpu.userbenchmark.com/Compare/Intel-Core-i7-2600-vs-Intel-Core-i3-10100F/620vs4082
I'm not sure what power supply you have, but if it's not going to fit just get a new one. Newegg has whole lot of power supplies on sale for black Friday, $50 - $70 is enough to get some of the best power supply on the markets from seasonic/superflower, and $40 will do if you don't care about brands.
At current black friday pricing, many parts are going for history low. It would be a great option to upgrade to 10100f($70 microcenter) + b560a-asus prime ($90 microcenter) + ddr4 3200mhz 2x8 gb ram kit(~$50 newegg) + decent 550-750w psu from sesonic/superflower (~$50-$70 newegg). The total cost is still under $300 while greatly boost both performance and further upgradability. Plus you can sell your 2600 and ddr3 ram to cover some of the cost. From what I see it would be much better option than go on ebay or trade forums to find some second hand old H61 motherboards that no one knows if it is reliable.
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