Though our two main PCs are dual core Athlon X2's we still have a P4 630 @ 3 gigahertz with hyperthreading and a P4 Northwood @ 2.8 gigahertz without hyperthreading up and running. The Northwood is equal to the 630 in single threaded games, the Prescott in anything that takes advantage of hyperthreading.
The 3.4 Northwood is worth $30 for you. It has hyperthreading just like your Prescott. Clock for clock, a Northwood runs cooler than a Prescott, but there's enough of a difference in clock between the two choices you have that temps might remain the same. Make sure your board's socket 478 and not LGA 775, as the Northwood's were all socket 478.
If you have the right socket, just make sure your bios supports the faster processor. For example, my i865PERL socket 478 board supports up to a 3.4 Northwood, but some boards might only support up to 3.0 or 3.2 in clockspeed. Check your board's CPU list online to make sure you have the right bios and board revision for that Northwood.