Been a bit of a while for me as well:
Mobiles overclocked can come close to stock A64 chips (just under a A643200 IIRC). There was a site that did this comparison, but I do not recall what it was. That was about where things stood though.
A
mobile 2200 and a decent motherboard should keep you under $140 (
Shuttle AN35N-Ultra
If you want to go A64, then yes, 754 is the way to go. I believe many in the CPU/OC forum were recommending the
Chaintech VNF3-250 motherboard.
The next question is which 754 processor... A lot of folks are commenting on the overclockability (including Anandtech reviews) of the
Socket 754 Sempron processors. They are currently running about $10 below the main running A64 overclocker which is the
A64 2800. IMHO, go for the A64 2800 over the Sempron at this time, the price diff is still just not justifiable at this time.
Memory, CAS latency on memory is not as big of a deal on the A64 as it is on the AXP. So you may be able to use your current RAM (with upgrade to get to 1GB if necessary), whereas you will definitely notice a big different with good ram on the AXP and may therefore have to ugprade all of your ram.
Lastly, if you are thinking of the overclock, then I cannot stress enough how important it is to buy a good PSU. E.g.,
Antec True 430
Good luck!