Bump for update: also posted in OP.
UPDATE: As I said I would, I have finally had the chance to follow the above steps. I can confirm that the guide can and does work. I'm now pulling 500Kb/s (estimated) download speeds over my wireless network, which is known to be slower than wired here. Either way, it's tons faster than I was getting previous over wired at home. I'll be able to test the wired speeds tomorrow.
However, I had problems getting my PS3 to connect to the proxy on Vista. Maybe I choose a bad static IP or something. I tried everything (disable firewall, for one), but simply couldn't get it to work with Vista. I really don't know what it is. However, it worked right away with XP. Set the computer to a static IP, download Privoxy and set it up (takes 30 seconds). Restart the computer (I've heard that can be a problem if you don't), start Privoxy...connect to proxy with PS3. Just like the guide said.
I'm wondering if Vista didn't work because it can use IPv6? I know XP doesn't...hm.
Anyway, if you haven't tried it yet, do it! Once you can get it connected, which is quick and easy, you should notice a difference if you aren't already pulling 500Kb/s-1Mb/s download speeds (though it may be even faster if you have a REALLY fast network).
I'm still waiting on that other modem, so I'll keep this updated.