I have a 2008 iMac with (I think) 16Gb of RAM which is used for just Firefox. I've been meaning to upgrade it to Linux but that generation didn't boot from USB, need to burn a CD.

All our intel MacBooks now run Linux just fine. The oldest is 2012, with 4Gb but most are 8 or 16Gb.

I would always recommend more RAM first over a faster processor; back when I would build desktop machines for Windows, I would use the second best CPU and put the savings into RAM.