Looking to upgrade an older box that has an Asus P4S8X-MX mobo.
That'd be the Intel® Pentium® 4 Processor Extreme Edition 3.40 GHz (Gallatin) 3.4 GHz, 800 FSB, HT.
Now this CPU is also "extreme" in price (i've seen offers for $495) and since it's such an old computer i wouldn't invest too much money. The Intel® Pentium® 4 530 (Prescott) 3.0GHz HT, 1MB L2 for $80 looks rather reasonable.
Also keep in mind, these high-powered Prescott and Gallatin CPUs require decent cooling, so if the original CPU was some Celeron, the original heatsink and fan may not suffice.
Check the list of all supported CPUs on the Asus P4S8X-MX here.