For a heatsink I would reccomend a Thermaltake Volcano 9. I had one with my old XP2100+ and it was valiant. You don't need a hot rod heatsink if you're not going to try overclocking (which I am NOT reccomending). With a Volcano 9, you can either have it run almost silent for everyday use, loud as a vacuum cleaner if you want to keep your CPU alot cooler (overclocking), or anywhere in between to suit your needs. That's what I loved about the Volcano 9. Plus it's only like $22 for an excellent quality heatsink. If you don't want to spend $22 on a heatsink, you can feel completely comfortable buying either one of
these (rated for the XP3000+ and above or even
this (rated for the XP2600+ and above. It's really your choice, however much you want to spend on a heatsink. The SLK900 and the Volcano 11+ are both excellent heatsinks, but I simply can't reccomend them to someone who doesn't say they want to overclock.
For a PSU, an Enermax 350W should be good, with plenty of headroom if you plan on sticking a bunch of hard drives, fans, or other stuff into your computer. Enermax is currently my favorite PSU company, then Antec and so on. 550W is completely unnecessary and frivolous for the average user, someone who doesn't have 10 hard drives and 18 case fans. I have 5 hard drives and 7 case fans and everything is chipper with an Enermax 350W.
For a case, I can't really reccomend one, because they come in so many completely different flavors it's impossible to tell what someone would like. Take a look around a computer retailer and pick one you like. If it's too expensive at the store, jot down the model name and find it online (but remember to add like $20 to an online retailer's price for shipping the case).
