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)