GB emulation is perfected. Roms are easy to find and s imple google search will have several websites with them.
When I find a good site i'll post it up. As for the emulators, almost anyone will work. Just go to zophar.net
and find and emulator or two that you like. Almost all games are playable without bugs.
GBA emulation is more recent. The ROMs are almost impossible to find. One major ROM site was recently shut
should cover most of the ROMs though. The emulation isn't perfect either. Some good emulators include Boycott
Advanced, Visual Boy advanced, and no$gba. All these emulators work fine with some bugs and glitches.