you can get an Athlon 600 for $180, an FIC SD11 Athlon mobo for $91 and a GFD for $50 (Northwind)
it's over 300, but with the GFD, you'll probably get the Athlon to 750 or 800 without endangering your peripherals with high bus speed... the only thing though is that you'll need some pretty good cooling if you want to get up to 800 (like an Alpha P7-125)
Take your money buy an Abit Be6 (or Bf6 or Be6-2) For 85-115 bucks. Buy a celeron 366 or 400 (50-70 bucks) clock the hell out of it and wait for the price of the cumines to drop. So you spend about a $150 and get a quick cpu that is easily upgradeable. Hey I enjoy my setup very much.
Celeron 366 running at [email protected] volts, with Golden Orb cooling that bad boy
v3 2000 at 175mhz
I would say an Athlon 600 and the FIC, or The K7Pro from MSI, depending on what it sells for there.
No GFD. Then you get a pretty good 600Mhz system - once you save the extra $50, you should have an instant upgrade to 750 - if you invest another $30 in a really good heatsink, you should make 800Mhz.
Grand total cost would be around $270 for 600Mhz, $320 for 700Mhz (maybe 750) and $350 for 800Mhz.
I am not the best person to ask about PIII - I know how much they cost and they perform well, but I do not know how much it costs (time & money) to overclock them, but Athlons are pretty easy.