A Scythe Ninja ought to be able to cope with a Core 2 Duo,
you can usually set the BIOS up to run the fan at variable speed as well. I'm assuming the PC case has fans in it as well?
( Moon Girl Technologies horrendous beanbag, Sun 31 May 2009, 21:58, archived)

yup, switched off that Q-fan crapola, case is old and stuffed with hard drives so that needs replacing too
I'll go and shovel some more money into Micro Direct next week
( Gilgamesh gazed in wonder as Frampton came alive, Sun 31 May 2009, 22:00, archived)

You'll want a case that'll take 120mm fans all round,
the bigger the fan, the slower it needs to spin, so 120mm is now the quiet computing standard. You can get gaming cases with 240mm fans in the side, but that's just daft.

Scythe and Noctua do the best fans, you're looking at £12-14 each.
( Moon Girl Technologies horrendous beanbag, Sun 31 May 2009, 22:03, archived)