Well, GBA emulation don't require that much of a machine power, unless u want great graphics. I was able to play Castlevania: Aria of Sorrow at full speed on my old Pentium 3 550, and I had an onboard video card!
The fastest config would be: -Turn off VSYNC; -Use the DIRECT DRAW RENDER METHOD, its the fastest one; -Since the Geforce cards have a fast memory, use the DDRAW USE VIDEO MEMORY; -Use the SUPER2XSAL filter; -Use Fullscreen mode. I like the 800x600 res, but 640x480 is also good.; -DON'T use the FULLSCREEN STRETCH TO FIT option. It will be ugly and slow.
-If u still get slowdowns, turn frameskip on. But don't use auto frameskip. Manually select the one u want. On my old comp (the p3 above), I used frameskip 2. On my current rig, I use frameskip 0.
One more thing: Remember that GBAs have a VERY small screen. When u stretch is, the graphics WILL look ugly. The use of a better filter can help a bit, but will make the emulation slower. If u want great graphics, use the real GBA, or use the unstretched screen. BTW, Here's a pic of a game running with the above config: