Your numbers are off a little... You must mean the A7V333 which has the somewhat older Via KT333 chipset, and the A7N8X based on the brand new nForce2 chipset.
Both boards would work fine with an XP2100+ Athlon, but the nForce2 would be faster.
My recommendation would be to go with the nForce2 board and if you can afford it, get one of the Athlon Thoroughbred processors (XP2400+ or higher). And again, if you can afford it, think about getting one of the new 333 MHz. front side bus Athlons. They are very pricy though, and right now I probably wouldn't pay it till they come down some.
As for your case question, for Pete's sake, don't buy some $29 generic case! Get a strong power supply, 350 watt or greater. I like Enermax, but there are many other quality power supplies out there. It's money well spent!
I don't know much about the AMD chipsets. So nothing on that.
As per the Case, definitely a big NO for those cheap cases. Not even if they boast their power supply is rated at 450W or more! If you don't want the hasle of getting the case and psu separately, get one from Antec's Performance Plus AMG series(the minitower model was enough for me, it comes with a 330W Antec Trupower psu).
On the other hand if you decide to get psu and case separately, you have a wider choice. Antec and Enermax make good PSU, just get something 350W or more. The case is up to you.
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