There are several vendors, who ship systems with Debian or other distributions of GNU/Linux pre-installed. You might pay more for the privilege, but it does buy a level of peace of mind, since you can be sure that the hardware is well-supported by GNU/Linux.
Whether or not you are purchasing a system with Linux bundled, or even a used system, it is still important to check that your hardware is supported by the Linux kernel. Check if your hardware is listed in the references found above. Let your salesperson (if any) know that you're shopping for a Linux system. Support Linux-friendly hardware vendors.
Some hardware manufacturers simply won't tell us how to write drivers for their hardware. Others won't allow us access to the documentation without a non-disclosure agreement that would prevent us from releasing the Linux source code.
Since we haven't been granted access to the documentation on these devices, they simply won't work under Linux. You can help by asking the manufacturers of such hardware to release the documentation. If enough people ask, they will realize that the free software community is an important market.
If you ask for Parity RAM in a computer store, you'll probably get virtual parity memory modules instead of true parity ones. Virtual parity SIMMs can often (but not always) be distinguished because they only have one more chip than an equivalent non-parity SIMM, and that one extra chip is smaller than all the others. Virtual-parity SIMMs work exactly like non-parity memory. They can't tell you when you have a single-bit RAM error the way true-parity SIMMs do in a motherboard that implements parity. Don't ever pay more for a virtual-parity SIMM than a non-parity one. Do expect to pay a little more for true-parity SIMMs, because you are actually buying one extra bit of memory for every 8 bits.
If you want complete information on PA-RISC RAM issues, and what is the best RAM to buy, see the PC Hardware FAQ.