Iono, I run dual view on a 7800GTX across (2) 22" monitors and don't really feel hampered by the performance drop, maybe your card is just insufficient?

I'd also considering using CT View port mod for that setup. keep your game area constrained to your 22" and put buttons, etc. on the 17".

Although really, I'd be more tempted to just run WoW on the 22" and leave the 17" for surfing (Thottbot, wow wiki, dual-boxing.com, etc.) The heterogeneous setup is kind of annoying.

that's my $0.02