Have you tried a different 802.11G access point / wifi router?
Standard 802.11g is pretty slow. You shouldn't have trouble getting an avi over to it though, unless your media server is transcoding it to something else, and upping the bitrate.
Nothing beats a gigabit wired connection ... it's .. nice ....
However, if you really don't want wires .. you can:
Get two 802.11G 108Mb/s access points and configure one as a bridge/client, and plug it into the PS3's Ethernet port, then the access point goes to your PC/network.
Alternately, do the above, but with wireless 802.11N.
I can't emphasise this enough .. a gigabit wired connection mops the floor with all wifi technology. Even a 100Mb wired connection will kill an 802.11n connection for actual throughput.
On an 802.11n, you're lucky to stream a heavily compressed 30Mb/s Hidef video stream without the occasionaly dropout (usually due to slightly higher bitrate required) .. whereas gigabit will get a decent 120Mb/s stream (that's about 3x the bitrate of a standard Bluray).
If you're having trouble streaming avi's to your PS3 across wireless, may I suggest that a second hand rug might be cheaper than two wifi access points. You can throw the rug across the cat5 cable.
The uverse Elite 18Mb/s is your internet connection speed, not your wifi router speed. Don't confuse the two.
You have an 18Mb/s (incoming) speed from the internet - A Wide Area Network.
802.11G/802.11N/Gigabit is a Local Area Network, and only refers to inside your house ... and possibly outside your house if you don't secure your wifi, and an enterprising bastard starts using your internet.
Streaming from your PC to your PS3 is using the Local Area Network only .. the Uverse elite 18Mb/s connection doesn't even come into it.