One of the most valuable tools in the PC hardware technician's bag is Memtest86+, a program whose primary purpose is to find intermittent errors in PC memory. It's a great way to burn - in new hardware and confirms it works as expected.
You can download Memtest86+ directly from http://www.memtest.org/ and create a bootable CD to run the program. Note that it's now included as one of the boot options for many Linux distributions, both on the installer media and when starting the OS normally. Any Ubuntu installer CD, for example, will include the Memtest86+ current when that version was released. Generally, if the program does not fully recognize your CPU hardware information, you probably need a newer version to make sure you're getting accurate results.
Once you've started it from a bootable CD, Memtest86+ reports the speed of the various types of memory at each of the cache levels it identifies. The most useful one is the MBps value reported on the memory: line. This will be how fast memory access to the main system RAM is happening, and is good for confirming that performance matches baselines. It can be a quick way to confirm performance and stability if you're adjusting clocking on your memory or CPU to try and speed things up.
One problem with this program is that it requires physical access to the system to run, which isn't always practical. Since Memtest86+ is only running a single process to test memory, its reported total speed number isn't really representative of the capabilities of modern processors. For that you'll need STREAM, which runs easily from a remote session, too.