I think the bigger gain by the removal of the 3 gig limit is on the servers themselves as they can take advantage of the memory on modern hardware.
The client and the server are two separate applications so it doesn't matter if they client was 64 bit, 32 bit or whatever.
What matters more, if you care about being visually impressive, is that the devs can now assume that everyone has a dx11 compliant GPU for the next expansion... One thing, is being able to address more memory. Being able to actually "show it", is where dx11 comes in...
All Gnomes are 2-bit crooks, schemers & megalomaniacs the other 62 bits are utterly wasted on such beings.