And I'm guessing that your HP pavillion has a Baby-AT case, so you will need a new case to fit an ATX board, and your powersupply is probably about 150watts, so you need a new powersupply...
You can find all of this at
newegg.com BIOSTAR M7VKD AMD VIA KT133A VIV 686-B 266MHZ FSB SOCKET A ATX - RETAIL - $64
AMD Duron 800MHz Socket A PGA Processor-OEM Version - $39
KINGSTON VALUERAM 128MB PC133 SDRAM - OEM - $28
Siluro Nvidia Geforce2 Mx200 32mb - $44
CREATIVE LABS SOUND BLASTER LIVE! 5.1 - OEM - $32
Cooler Master DP5-5G11A AMD Socket A Processor Fan with Heatsink - $5
EVERCASE E4252WEF25, Intel P4 / AMD, 10-Bay, 300W P4, Truely Screwless ATX MID Tower Case. - $40
I don't think I missed anything (though you might want to add a case fan for another $5)... So...
Grand Total: $252 - That leaves you with $23 for shipping.
Now, I know NOTHING about any of these products, and am making no implication that this setup wouldn't be a real pain in the ass. I'm also not saying it WOULD be a pain in the ass. All I'm saying is that this is the list of parts you need, and this gets you in under your budget.