here's some answers
1.win95 is good but not as stabe as 98se which I have found to be fast and stable on my last 4 PC's I had 95 before and it was good but 98se is better ME is actually very good when you look at the base of it and will take over from 98 however it is still on early release versions so dont go for it yet it is FULL of bugs. Oh and I dont like 2000 it does strange things and microsoft have decided people should have a hell of a time finding anything they want in it.
2. I have a Athlon and I would say use 300W as a minimum you will get rid of all the random strange acts and crashes that happen with lower PSU's however if you afford a 350W monster go for it a nice extra chunk of power always helps especially if you don't know how much you will be bolting onto it later (and trust me if you are anything like a normal person who builds PC's you will bolt a LOT on)
3. Most motherboards that monitor heat either have built in to their bios or have with bundled software something that will warn you of the temp getting to high with my pc it starts beeping like mad and then shuts down if it gets too much hotter. This can usually be set up in the BIOS I can't garauntee the ASUS has this but the likelyhood is it does.
anyone got an ASUS that can help out here as to its BIOS options?
4. The memory is compatible. CAS is the delay of the chip while the PC looks for the correct part of the memory module to use the lower the CAS number the quicker the RAM you won't notice much difference between CAS 2 and CAS 3 however but CAS 2 has better future compatibility with higher bus speeds (whether you will still be using the same memory modules then is a different question though.
5. Its all about the amount of noise they make and how long they last but your power supply and hard drives are probably noisier than the processor fan and both sleeve and bb fans last for flippin' ages usually anyway.
Get the one recommended by AMD at
http://www1.amd.com/products/athlon/thermals
6. that boot disk should be fine especially if you are building 95 doesn't really matter too much as long as it has some cd drivers and a decent version of DOS if you are just installing windows. I usually build win98 off an old win95 boot disk.
7. The tbird is stunning definately my recommendation can't tell you about the m/board though but I expect its pretty good I personally use a ABIT KT7 very good board make sure you have the latest VIA drivers if you go for the KT7 though.
Hope that helps