While on my lunch-time browse, I found this over on The Inquirer which points to a German site GameStar.de, which (you following this...) eventually pointed to a Japanese site where some enthusiasts have overclocked a Pentium IV 2.2GHz to a whopping 3.5GHz, using some nitrogen cooling.
The specs for this test (which ran for a "short time") were :- Abit TH7-II, 128MB of memory and an ancient Matrox Millennium, so that no other components influence the experiment negatively.
The site (in Japanese) has all the details, including some screenshots of the whopping internal clock figure of 3541.87 MHz as well as a photo of the cooling used.
Also listed on Holicho's site is a benchmark figure using 3DMark2001 of 10675 3D marks. This figure was obtained by overclocking a Pentium IV 2.2GHz to 2.9GHz with some rather large cooling... Take a look for yourself!
News source: Holicho's Homepage