I agree with the estimate that you'll get about two years of gaming out of it, maybe a little less or more depending on whether you upgrade the graphics card along the way and the kind of games you like.
For everything else you mention, it'll easily be good for another five years, probably ten if it still runs at all.
Though more interesting things have been and will be added to the internet, the requirements for web page viewing, e-mail sending, and the occasional light flash animation and video clip haven't changed significantly in several years, and I don't see them going up drastically in the near future. If anything, web browsers have gotten lighter weight and more efficient in the past few years, not less--look at Safari compared to IE5 (which was the best out there in 2001).
Also keep in mind that the rate of speed increases on processors and RAM has leveled off quite a bit, in part because most people outside media pros and gamers just don't need more than a ~1GHz processor and half a gig of RAM to do anything they want. I don't see that changing for any good reason in the near future.
By way of example: Where I work we have a set of 400MHz first-gen G4s; it has now been 5.5 years since they were released, and with some extra RAM and a hard drive upgrade ($200 max), they run 10.3 fine and they're perfectly functional office machines for web surfing, word processing, excel, and even data analysis (plus the occasional video).
Your G5 is roughly the 2004 equivalent of this, so I see no reason it won't get the job done for the next 5-10 years unless you start wanting to do something fancier.
Heck, even a first-gen, bottom-of-the-line iMac, coming up on seven years old, will run OSX and get you around on the internet, albeit slowly.
I also concurr that it'll be a long time before you're likely to need more than 2GB of RAM--I hammer on my G5, and the 2.5GB I've got is still plenty.
