System Admin - Hardware
Determining a 32 vs. a 64 bit kernel
See System Admin/nix#32vs64bitkernel
Failing RAM is really rather painful and sometimes difficult to detect / diagnose. If a system (particularly a new one) starts to misbehave at high loads a memory test would be a good idea.
Here's a couple of tools which I have found useful for diagnosing system memory:
Memtest86 (http://www.memtest86.com/) - Thorough self-booting memory tester
Memtest86+ (http://www.memtest.org/) - fork of Memtest86
Memtest (http://hcidesign.com/memtest/) - Windows-based runtime test. This tool really needs to be run in Safe Mode otherwise false errors are reported.