I play with ~100 ping and its fine for me. Really, the thing that matters is smooth ping. Sure, lower pings are more beneficial because you dont ahve to predict as much, but as long as the ping is consistent you should be able to get pretty accurate.
The thing that affects ping the most beyond connection speed is the distance you are from the server, but in the US you should be able to ping the opposite coast with a ~100-120 ping. In Q3 you can increase the snaps value to 40 or so with the console command "\snaps 40" and increase the packet throughput with "\cl_maxpackets 120" to give yourself a smoother connection.
A 1.25GHz G4 should run the game fine, depending on the video card you have, of course. The video settings you use are up to you, but if you find the game to be a bit choppy, you can turn down the resolution as well as turn off lightmap and trilinear filtering to give yourself a boost in performance.
If you're new to the game you'll just have to get used to the gameplay. Go play some bots and such in singleplayer to get accustomed to the controls and how the game functions (and how effective each weapon is) before worrying about dieing too much in online games.
Additionally, go grab gameranger at http://www.gameranger.com and set it up to search for quake3 servers. You can list them by ping so you will get servers that are both populated as well as have a low ping for you.
you can reduce ping by fine tuning your cfg file (a bit) I cant remember the details, but google the upset chaps quake 3 guide - it has the good oil for sure.
It also advises on how to improve your scores by using vertex lighting, removing shadows etc - eg low detail rather than high details. If your trying to play osp with 100+ ping, its no wonder you struggle, if you cant reduce your ping, download some unlagged mods, and play them, youll have much less prediction to do and have a more even playing field.