Running Arch on an overclocked PC is totally fine provided that the PC is stable at the overclock settings. There are several programs available to you that will help you stress test your system. The steps of overclocking a PC are beyond the scope of this article, but there is pretty inclusive guide written by graysky on the topic you can read at your leisure.
A very good program for stress testing your memory is [Memtest86+]. It is based on the well-known original memtest86 written by Chris Brady. Memtest86+ is, like the original, released under the terms of the Gnu Public License (GPL). No restrictions for use, private or commercial exist other than the ones mentioned in the Gnu Public License (GPL).
You may download it from the webhost of memtest86+ [here] either as a bootable CD ISO or as an pre-compiled bootable binary. The later can be called by GRUB with a minor modification to your menu.lst to allow you to boot directly into Memtest86+ without a CD/DVDROM.
Either download and burn the ISO to a CD and boot from it, or follow the instructions in the next section to add an entry to your GRUB boot menu. Either way when you enter Memtest86+, the application begins testing your memory without your intervention. It will run indefinitely until you stop it reporting any errors as it goes. When it has completed a number of iterations without errors or runs for an arbitrary amount of time without errors, you can pretty much call your memory "good" or "stable" at the settings you have chosen for it in your BIOS.
Stressing CPU and/or Memory
A very good program for CPU and CPU/memory stress testing is [prime95]. There are both x86 and x86_64 version for Linux you can freely use for stress testing purposes under Linux.