I am an experienced PC builder, and I am also on the market for a new Thunderbird system. To eliminate heatsink woes, I have decided to pay a few extra bucks (actually under 30, price of a good HS/Fan combo) and buy a RETAIL boxed Thunderbird. It includes a 3 year AMD warranty (OEM parts usually come with 30 days to 1 year and hold no warranty with AMD) You could get a better cooler for the money, however, there is no thermal protection built into AMD Tbird CPU's and an improper installation could burn up the chip and leave you at the mercy of your vendor's warranty (which may not apply in this case).
Some other considerations buying an AMD system:
-Make SURE your power supply and memory are AMD approved (check their website to be sure) and of good quality. I have seen some vendors sell cases that are AMD approved with a power supply that is not. Be careful. Antec makes exellent cases w/ AMD approved power supplies that will cost between 75-120 dollars w/ shipping. Mushkin, Corsair, and Micron/Crucial are all good memory brands that work with AMD - go PC133 for future headroom, PC100 if you are on a budget is fine also.
-Motherboard choice is critital, do not skimp. A lot of people like the Asus A7V or Abit KT7. I am going with the MSI-Microstar KT7 Pro2A. <based on stability, performance, and price per newsgroup reading/web reviews versus the other two>
-Hardware compatibility problems exist with equal frequency in Intel/AMD based systems and are usually due to cheap peripherals or poor driver support, NOT the CPU. A big reason motherboard choice is critical in any system. Stick to brand names and do a bit of research if you are not sure about a particular product.
I suggest you assemble a list of the components you want to use and post it on here somewhere, see what suggestions people have. I also suggest you run the name of the supplier(s) you intend to use in the newsgroups and see if people know of them. I REALLY like www.pcprogress.com for most components and have ordered from them often. They are not always the cheapest, but manage to stay close and charge reasonably for shipping.
Good news, btw, I just noted that there was a price cut on AMD chips, saw OEM 1-ghz Tbirds for UNDER 200 bucks for OEM models (226 was lowest on Retail)
Good luck!