It's fused 4A at 117, so if you plan on running it flat-out at any stage, you'll want at least (4x117) = 480watts (ie 500)
The amp has a slo-blo fuse, so I guess the designers anticipate it to momentarily draw more power than 480watts and anyway, it's a good idea to go a little bigger than you need if you can afford it.
I'd go with a 800watter. The bigger they get, the cooler they'll run, and the longer they'll last you. I use a 1000watt transformer with my MKIV.
