Well, I tend to use my computers until I am forced for an upgrade. Before I got my generic Windows 98 Celeron 400mhz 328mb ram computer around 1999 (which I am using right now), I had an old 486 Packard Bell, which was well used at its time. So I might end up going farther than 4-5 years with my next machine.
Mom is helping pay, thankfully, but only for the laser printer and taxes. I pay for the rest and only have $2000 myself. In fact, I'll probably have to use my current 17in. CRT monitor for a little while, until I can afford a good ol' cheap LCD. This is if I go the PowerMac route. (Couldn't I get a super nice CRT for cheap that could take advantage of the video card?

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In addition to the basics (writing papers and stories, email, Internet, voice recording, building a website of sorts, downloading and listening to music, etc.), I would also do my extensive project of digitizing all the family photos and videos for preservation and eventual export to DVD.
I still like the PowerMac because I can upgrade the video card and just about most other things. However, I question that I would ever change anything besides the harddrive and ram, since I am not a gamer and don't change my video card monthly. Also, I would have to buy a microphone (voice recording is a must), better speakers, monitor, etc. Yeah, that iMac is starting to look better and better. Shame you can't keep extending that warranty forever!