The stock heatsink is excellent when not overclocking. I wouldn't pay top dollar for serial ata hardrives. The net speed doesn't even approach 150. I would suggest a standard ide western digital or maxtor ata 133 with 8 meg cache if the price is significantly lower than the seagate. Can't advise you on the memory or video card, but you're right; some people have had issues with both. I would buy from a good vender such as newegg, in case you have any problems and need to return them. The power supply is more than adequate. I would also try the onboard sound before getting a premium soundcard; it may work just fine.
Maxtor is the only hardrive manufacturer with the ATA-133 IDE & SATA-150 interface. No problems with 1Gig of RAM with WinXP or Win2k, problems arise when they try to run 1Gig of RAM, of any kind, with Win98,98SE,or ME, those OP/SYS's only effeciently run 512Mb of RAM. Corsair is excellent high quality RAM. And the heatsink will be sufficient, but I'd go with the Vantec Aeroflow for the P4, its smooth,quiet, and cools very nicely.