I can't answer all your questions, but I can answer a few...
First, you may want to take a look at the Hardware Compatibility List
My 2 cents on the hardware in question...
AMD Athlon 64- yes you can use it. my 64 3000+ runs like a champ.
MSI K8N Ultra SLI mainboard- Should be ok, but not 100% sure.
ASUS Geforce 6600 pcie VGA card- I'm currently using a Chaintech Gforce 6600GT PCIe and it works fine with lastest drivers.
Sata disk drive- Alot of folks are having problems installling onto SATA. I never could get SuSE installed on a SATA, but it was detected after I ran SuSE's updates. It's currently being used as storage space.
IDE and SATA hard drives together- yes, you can run both of them if your motherboard supports them both. I currently have 2 IDE's and 1 SATA in my main box. I'm guessing you're plan was to install your preferred distro on the SATA and use the IDE as storage?(see above)