Well after many hours spent researching this problem, I finally fixed it. Like you all, after updating an old mac from 10.5.8 to 10.6.3 (and then .8 as part of my attempts at a solution) my airport was basically dead. I could only load google and even that was slow and only possible sometimes.
Anyway, for the longest time I assumed it was some sort of setting I needed to change on my computer--after all, that was what had changed. I did briefly fiddle with the wireless settings but everything was as recommended so I didn't spend too much time there.
What finally worked for me was resetting my **** router settings. Why I didn't think to do this before is beyond me--it is so simple.
The problem, I think, was the router channel. I noticed that the Region field in the router settings was set to "Europe" which apparently corresponds to a different set of frequencies. Also, in the system profiler, I saw that some channels (specifically 12-14) were not supported by my airport card (while I think it is standard in Europe to use those channels). My router would not let me change the region manually but resetting all settings to factory default took care of that (I think I could have manually assigned a channel too).
Very glad to be done with that. I feel like such a fool to have neglected to try such an obvious solution from the start, but I'm glad it's over. By the way, I'm referring to the router settings accessible on the router management portal accessible by typing 192.168.1.1 or some other IP address into your address (I don't know what that IP corresponds to cause I think that that's the same for all Netgear routers or something). Anyway, good luck with your problems!
More generally, I would suggest to just look through all your router settings and make sure nothing is amiss. If you don't care about preserving your settings I would go straight for the hard reset.