How do I watch or stream movies over WiFi without constant stuttering?

You could try using VLC to do streaming, that might cope better

Set up a decent Media Streaming application. XBMC edition is easy to use and buffers decently! Have UPNP on your router turned on!

What kind of wireless card/adapter are you using? I went through 5 before ending with a slightly older Intel 4965AGN before i got perfect streaming. Broadcom WiFi chips seem to be quite bad at streaming. I even went up to Broadcom draft N chips and they didn't help any over G. Try finding an Intel wireless card if you're using a laptop. Fact is there are so many variables with where you live, it can be very hard to figure out what could be the main problem. The final form of N should help a lot as with beamforming.
What kind of wireless card/adapter are you using? I went through 5 before ending with a slightly older Intel 4965AGN before i got perfect streaming. Broadcom WiFi chips seem to be quite bad at streaming. I even went up to Broadcom draft N chips and they didn't help any over G. Try finding an Intel wireless card if you're using a laptop. Fact is there are so many variables with where you live, it can be very hard to figure out what could be the main problem. The final form of N should help a lot as with beamforming.

My home setup streams TVersity to my PS3 over 802.11g. The media server computer is like a PIII 800Mhz w/ 512MB ram. I'm able to stream w/o stuttering. Try it!

What standard of WiFi are you using? The newer 802.11n is much faster than the (more common) older 802.11g protocol and should be able to stream your videos (and maybe even HD) at sufficient speeds.