I'm pretty sure that you'll overclock that Duron somewhere down the road (immediately after the first boot?

) So let me say that you should stay away from the CoolerMaster HSFs. I bought a 1Gig TBird/HSF combo from an online retailer. Even at stock speeds it ran hot. The fan on most CoolerMaster HSF's is only rated at 16cfm. I'm sure that this would be a nice HSF for say, a 600 Duron that wasn't going to be overclocked (yeah, right!) but it just doesn't cut the mustard.
Go with a FOP-32-1. That's what I use now. Make sure you use some Arctic Silver/Arctic Silver II thermal compound...it makes a big difference. Don't skimp on the CPU HSF, you'll thank yourself later for spending a few extra bux. As far as case recommendations go...I'm really not all that well-versed in cases. But there are a few things that I can tell you to look for.
1. Most importantly, make sure the case's power supply is on AMD's Approved PS list. Durons/Athlons suck a lot of juice and a generic ps will not run your system well, if at all. 300watts is the minimum. If you get a quality 300w ps (Antech/Sparkle just two names that come to mind) you'll be good to go.
2. The case should have good ventilation. At least two fans, or spots for fans, one in front at the bottom and the other on the rear panel in the middle/top and you'll be fine.
3. Make sure the case will be big enough to support future upgrades. Enough 5.25 bays and things of that nature.
good luck.