I haven't tried it personally, but apparently FFXII is one of the better running games (I think). I've seen screens at least and it's pretty awesome looking. The emulator is still in a kind of unpolished state, both in functionality and user interface, but it does the job and it's not at the point where it's overcomplicated like epsxe (yet). They're more worried about compatibility rather than enhancement at this point. Dual core is pretty much a necessity though (although I think you're good there). I got Klonoa 2 running full speed, but there were a few graphical errors here or there.
In somewhat related news, I was messing around with Gamecube emulation today, and it's coming along. Wind Waker was fully playable from what I saw, pretty close to full speed without much tweaking (it wasn't running quite that well for me, but I wasn't using the ideal version from what I understand). There is no sound however. Apparently, the main GC emulator - Dolphin, went open source rather recently and a lot of progress has been made (to the point where it's even running Wii games). It's nothing you can just sit down and play through completely at this point, but give it a year or so and I think we'll see big things. They're fulling taking advantage of x64 now, and that's helping a lot. I'm actually pretty impressed with what I've seen of the DC, PS2, and GC emulators in the last year or so.