I have just successfully configured my wireless network using the Netgear DG834Gv3 modem /accesspoint/ Router.
The networked PCs are the main desktop (XP SP2) and a DELL laptop(Vista Home basic)with builtin wireless adapter.
Folder/file sharing from the Vista lappy to the main Desktop(XP SP2) posed a little problem but after going through the Vista Help files and other articles, I got the right way and implemented it successfully.A seperate post will deal with it.
The basic setup is like this:
ADSL line after the splitter feeds the Netgear, with one of its LAN port directly connecting to the Main desktop ( using a normal cable - not a crossover cable).The Dell laptop(Vista) wirelessly connects to the Netgear.
I presume your final aim is to have such a setup as everybody would like it to be.The only difference in your case is, you have a seperate modem/router + accesspoint/ Router and there should be no problem in acheiving a wireless connectivity and network as in my case.
Your post is a little confusing but I presume that there is some conflict when using the modem/router + accesspoint/router which could have been avoided if you have used the modem/accesspoint/router(all in one) device.(No doubt your Buffalo is a widely acclaimed accesspoint/router)
You may have to reconfigure your modem/router to act only as a modem with other functions like PPPoE/DHCP/DNS/firewall taken care of by your Buffalo accesspoint/router.These are my first thoughts.
I have already downloaded your Buffalo's user manual and let me have the manufacturer/model number of your modem/router. If I could get its user manual from the internet, I may perhaps be able to analyse and suggest a solution.
I am not a networking expert but I have learnt quite a lot ( no help from Digiticians

) while successfully networking my PCs.

( I do not know honestly what is meant by razz but hope that fits in here)