My Mac is quite old (over 5 years now) and is still going strong, running the latest version of Tiger. So I can safely assume the your mac will not be obsolete in a few years time 🙂
yes, it will last. unlike PCs, Macs are much more resistant to "aging." For what it's worth, my 2 year old iMac performs exactly as it did the day i opened it.
so unless your needs drastically change, then it will last!
Five or six years should not be a problem. Although my current laptop is a C2D MacBook, until quite recently i was running OSx Tiger on a 12" G3 iBook.
Photoshop CS2 and other "new" software was very usable on this computer.