Older mass produced PC's seem to work flawlessly with Mandrake 9.2. I've installed old Dells, and my generic homebuilt P3's without any hardware issues. Mandrake 9.2 even took care of setting up and recompiling my kernal on the one system that had an NVidea video card.
Don't know what your budget is, but there are places like www.penguincomputing.com
(haven't used them) that will ship you a PC already loaded with Linux. Most of these outfits (I've been looking around myself) install the latest RedHat distros, but that's a pretty good acid test of the hardware. If RH is working on it, it would be very unlikely that Mandrake would not work as well.
If I were on a budget, I would go for refurbished IBM PC's because they have a Linux-ready evaluation on their website. Find them on ebay, then check the model number on the site for compatibility. They pretty much list any known driver issues there, so you would know whether a particular model will work. Dell is another possibility, since they use Intel motherboards.
I used the Mandrake site for their hardware compatibility list. You can check the components there and then order a barebones system from anywhere and be pretty safe. If it's a motherboard with everything built in, make sure it's certified.