Your plan to upgrade your database in place to 10g and then use Data Pump to migrate to another platform is a very good plan. In almost all test results I've seen, Data Pump is considerably faster than using export/import, although I cannot say how long this process will take since it is dependent on hardware and the type of data contained in your database.
Another option you may want to consider, if you're going to migrate in place, is the Oracle 10g cross-platform transportable tablespace feature. This feature will allow you to migrate from one platform to another. If you are running HPUX 11.0, 11.11 or 11.23 you can install Oracle 10gR2 on your HPUX server, upgrade your database in place, and then use the new cross-platform transportable tablespaces feature to move your database to your Linux server. Both HPUX and Linux are supported platforms (you can query the view V$TRANSPORTABLE_PLATFORM to see supported platforms) but they are of different endianness. HPUX is big endian while Linux is little endian, so there is an additional step involved to convert the tablespaces being transported on either the source or target side.
To read more about the cross-platform transportable tablespace feature, download the Oracle Database Administrator's Guide 10g (release 10.2) from Oracle Technology Network.
You should also review the Oracle Database 10g Release 2 (10.2) Upgrade Guide.
This was first published in May 2006