Possible it is possible, but if I was in your shoes I wouldnt do this to my laptop. First is not worth, a Intel Core 2 Duo T5500
alone costs about £.100, the CPU replacement is not a simple operation and should be performed by an expert repairman, this will cost you about £ 80 plus VAT labour, replacing cooling system with a better one would also be good.
Then after you replace it you risk to overheat your new CPU and blow it, this is one of the reasons why Acer tells customers not to upgrade to core 2 duo. Additionally, if you want to DIY you risk to damage your laptop on a complex cpu replacement operation.
The guy on you tube wanted to show his skills, but I am sure any genuine computer repair engineer will suggest you not to proceed with this kind of operation.
With all money required, labor, and risk involved in operation you will better buy a new laptop, if you really need a powerful laptop buy Alienware
, uk custom made supercomputers.
Regarding the memory I know that some customers successfully mounted 4 Gbs on 4315, while I also read on the Internet other 4315 did not support 4 Gb. I assume it depends on Bios version. I suggest to contact the memory seller posted in previous message and ask if 4Gb are supported, giving them exact model number from your laptop service tag, and also ask if you can eventually get refund if this does not work.