An i5 CPU will be perfectly fine for arcade emulation with MAME. These CPUs with the stock cooler and stock speed should be very quiet if you have even slightly decent airflow.
For smooth Wii emulation with Dolphin in great quality, you need a strong CPU and GPU. An i5 is good for the CPU, and a GT 430 should be OK with some graphical features lowered/disabled in most games. Just don't expect the best out of this configuration. However, regardless of what hardware you're using, prepare for headaches with some games.
Things might have changed, but none of the PS2 emulators were very useful with the vast majority of games when I tried them last year. I was not aware of any working Xbox emulators.
While MAME, 8-bit, 16-bit and PS1 emulation programs are a real joy to use, emulators for the recent systems either do not work or require more tweaking than most would care to bother with.
As for a better GPU recommendation, I'll leave that to someone else.