I upgraded my P3V4X from a Celeron 400 to a PIII 1GHz EB CPU (133MHz FSB).  CPU was $250 Canadian (retail box).  I watched this price for a looong time before buying and am somewhat pleased that it is going UP...after I purchased.
One thing to note is the released versions of the bios for P3V4X may not recognize the CPU correctly.  While I did not experience any errors running with, I think it was, the released 1005 bios, I did receive error message during startup.  I found on Asus web site and flashed (many fingers crossed) the 1006.02 beta bios....same error.  Then found 1006.003 beta bios on, I believe, german web site.  This worked (even more tense flashing experience) (although it still reports 1006.02 beta when booting).
Machine has been rock solid so far (about a month).
Luckilly I bought a good (Asus) slocket some time ago in prep. for upgrade....they I understand are not available or very scarce now.
Did have to search for a CPU heatsink that fit properly.  Retail unit was too wide (rested on the norhbridge chip).  Next one I tried was too tall.....hit the SDRAM.  Found a CoolerMaster unit that fits just fine.
While I did contemplate changing to an AMD based board, it was not really for me the best solution due to things like:
1)  My case, which I like very much (SuperPower), has a 250 Watt power supply.
2)  I do not like buying things that may not be best future upgrade solution and if I was going AMD would want a DDR board...so new ram.
(hey my P3V4X is 2-yrs old - and as far as I am concerned runs just fine)