it just depends. the 2500+ is only-- what, $90 now? you could get $75 for your 2400+ maybe on FS/FT or ebay. i don't know what your HSF looks like, but if you plan on oc'ing the 2500, it could be a nice gain for $25. also make sure yer mobo supports barton (bios update may be needed).
just buy a Athlon XP1700+ T-BRED DLT3C JIUHB at www.bigfootcomputers.com its a canadian site, but they have 6 of them in stock if you want one get one.
I bought mine from them, great cpu 1466mhz overclocked to 2300mhz = 3200+ with A nforce motherboard