Any harddrive you feel like. If you want RAID the best (this side of 300 dollars) solution is MD software RAID. The most important item to look out for is the drive controller compatability. The harddrive itself doesn't have any drivers it is all interfaced thru the controller, which is what the OS deals mostly with.
For SATA drive controller compatability check out
http://linux.yyz.us/sata/
It's very up to date, bleeding edge support, but most of it will work with the kernels provided by debian. Almost everything is supported. don't bother with 'sata' raid controllers unless they are very expensive though. If they claim to support 'raid' that's fine, but you'll only want to use them as a regular drive controller.
One thing to watch out, that I know about, for is the combination of Silicon Image 311x-based (3112) sata drive controllers and Seagate drives. This isn't due to Linux support directly.. it's a hardware issue with the drive controllers that only seems to affect certain seagate drives. There is a blacklist of drives that when used with the silicon controller that they'd operate with non-dma access, which is slow, in order to protect any data on the drives and the system stability. I think there are work arounds, but I don't know much about the aviable patches. I have 2 of these SATA to PCI adapters in my computer and they work fine with the drives I use. There are some motherboards that have them onboard.
When installing Debian Sarge using the netinstall cdrom... make sure that you have linux26 typed out at the boot prompt. By using the F buttons you can get more information. By doing this it sets up using Linux 2.6 series kernel by default and does some minor changes in the installation program. Otherwise it uses 2.4 series by default and if you want to switch to 2.6 after you install there are some small transitions that you have to go thru.