I actually wouldn't get rid of the old IDE HDD, I have the same HDD configuration.
As for whether the disks matter.. well they don't. Im using a 160GB IDE HDD dedicated to SL and a Windows 7 Ult. Edition x64-bit installation on my 1Tb disk, and not a single R/w error.
If you want working wireless, id recommend either looking into dedicated Mac hardware, or going to InsanelyMac's article on adapters that will work out of the box. Aka, Plug n' Play.
The main thing I would do is look for a compatible graphics card. The worse thing is to be stuck with a graphics card that's a fish out of water when it comes to Snow Leopard. I'm sure theirs an article about that somewhere too.
Never, ever, get a Nvidia GALAXY GT 240. Linux will hate you. Snow Leopard will too. It's been the root of all my Unix problems. If you don't need a snazzy new GPU, look into a i3 Clarkdale w/ GPU capabilities. I'm not sure about the PnP possibilities, but it might work.
Remember - CLARKDALE. Not Sandybridge.
Hope this helps, I just got my hackintosh off the ground, but i've messed with unix and windows systems for years now.