Frostytech have tested an Intel socket 775 heatsink that's under 50mm in height! See for yourself!

"The Nano Silencer TLI-U is ideally sized for 1U server chassis, nano formfactor PCs, car PCs and media center microATX-style cases. The entire TLI-U heatsink stands no taller than 45mm, and uses the same swagged aluminum fin technique as the Micro Silencer model."