Firewire is awesome for the attacker, unfortunately few servers have it, especially not externally exposed ports. Also, smart OSes use some of the newer Intel features (VT-d) to lock down DMA while the OS is running, which usually protects from rogue firewire, and can theoretically help against rogue PCIe, although usually badly implemented in chipset and OS.

Another option is a reboot onto a custom OS which is designed specifically to preserve memory (you get a safe few seconds of holdover). LiveKd is pretty cool (sysinternals)

There are PCIe cards which do processor/network and let you explore main memory -- WindowsSCOPE CaptureGUARD for PCIe or ExpressCard. Probably enough time to pop the case open and throw one in before memory degrades.

Countermeasures are numerous -- everything from doing memory encryption inside the CPU die (putting code in the cache, like TRESOR) and doing hypervisor tricks ("TresorVisor") (http://www1.informatik.uni-erlangen.de/tresor) to using Hardware Security Modules (like the SafeNet or Thales nCipher) to just keeping your servers physically secured from intruders who might memory-analyze them (although a software bootloader and remote-reboot could still be applied).

Forensics as a field seems to be a lot more interested in attacking mobile phones (which is one of the things I'm talking about at RSA 2013), but desktops and servers are still interesting targets.