I walked into the Arcade, long before the holidays patch, and have been treated to the most abhorrent framerates I've ever experienced in Spiral Knights. For the sake of real numbers, though, I will use the latest patch (as of 12/14/2011, 4:15 PST).
Let me set the stage:
Operating System: Windows 7 Ultimate 64-bit (6.1, Build 7600) (7600.win7_gdr.110622-1503)
System Manufacturer: INTEL
System Model: DX58SO
Processor: Intel(R) Core(TM) i7-920 @ 2.67GHz (8 CPUs), ~2.7GHz
Memory: 8192MB RAM
Available OS Memory: 8180MB RAM
Card name: NVIDIA GeForce GTX 470
Display Memory: 4064 MB