4-boxing will appreciate the 8GB, but probably never quite fill it. I can't speak as to its behavior with Wrath installed, but I doubt they would double the memory usage. Increased transfer rate OTOH might be nice with four clients trying to thrash it simultaneously whenever something needs loading.

Although with 16GB, there's an alternative path that opens up. The WoW data directory is currently about 8GB...and RAM drives trump the tar out of flash SSDs. But it's volatile memory, so you'd have to wait for it to reload all 8GB every time after you power down the machine.