I would personally say that your Windows 7 computer is a better and safer Internet citizen in general than your old PowerPC Mac, still running 10.4 or 10.8.
The danger with old, unpatched systems is rarely in somebody hacking in and getting your data. The danger is with somebody hacking in and using your resources for their particular nefarious purposes, which usually involve sending spam or phishing message. These days, there are
automated tools that can break into Linux and BSD systems. Given that Mac OS X is "just" a BSD system with a special display layer and windowing manager, it's not too crazy to believe that a Mac could be susceptible to these kinds of tools.
In addition, not only is it possible to execute malicious code on a Mac through Java or Flash (which you should try to avoid on Windows and on modern Macs as well) but it's possible to get malware onto a Mac by having a program just literally ask for permission. Most of this malware targets pretty new versions of Mac OS X, but there's no reason it couldn't target 10.6, 10.5, or 10.4. (This isn't just a "PowerPC" issue -- 10.5, 10.6, and 10.7 on Intel-based Macs are also insecure at this point.)
And, none of that talks to what everybody else in this thread did, which is that the built-in, "period" browsers for Mac OS X 10.4 and 10.5 on PowerPC are woefully deficient by today's encryption standards, so if you're say, using a PowerBook in your local Starbucks (which is probably possible, Apple has built good batteries for a really long time) it's possible that somebody will sniff your wifi traffic and capture the login sequence with your financial institution (or your school or work or whatever.)
There's stuff you can do even on old PPC Macs to mitigate part of that risk a little bit (such as using a VPN service or tunneling all of your web traffic over an SSH connection to your home system) but at that point you'd almost be doing as well to just run Firefox on X11 over SSH instead of running anything on your local Mac at all.
Other things to consider, just as general notes: Mac OS X has had IPv6 included and enabled by default for a really long time, and by default (to this day) the firewall is not enabled for IPv4 or IPv6 by default, so that will be a thing you'd want to look at.
For web browsing, using TenFourFox will help with the encryption cipher thing, but if your system's compromised somehow, all bets are off, and for as often as new vulnerabilities that affect Linux and UNIX these days show up, it's probably just best not to connect a PPC Mac running OS X to the Internet at all.
If you only want to have PPC Mac hardware, and your mail goal is to be able to securely use the Internet, your best bet is to put something like a current Linux or BSD on it. There's malware for those systems, but since they're still actively being patched, that problem is likely to be fixed sooner rather than later. (or, you know, "at all.")
and I can't get TFF to store it.
