It's really hard to settle on just one computer when you know a few months later something new is going to come out, and a few months after that, and a few months after that, and that, and that.... When should I put my foot down and actually get it?
While it's true that technology is leapfrogging ahead ever 6-8 months, I always tell my clients/friends to look at it this way: 2.4 Gh today will still be 2.4 Gh tomorrow, as will 2 Gigs of RAM. When C2D's came out, P4's weren't rendered completely obsolete overnight; the same holds true for Quads. Unless you run several multi-threaded tasks in the background *and* play the newest games or use programs that require insane amounts of CPU power and/or memory, stick with the basics.
I use a Pentium D 940, and have for a while now. Until I bought it, I was on a P4 running 2.4 Gh. Only reason I changed was because I wanted to build a new system just for the fun of it, and my sisters wanted a computer to do their homework on. It was a win/win situation. If not for their want of a system. I'd still be using my P4 and it would more than suffice.
You don't need to spend $1500 on a system to meet 'basic needs'. Further, a system that costs $500-$700 to build/purchase won't be completely obsolete within 3 years unless someone time travels back and hands society a warp core

. Look for a stable Dual-core CPU, around 1-2 Gigs of RAM (doesn't even have to be 800m, 533 will suffice for day-to-day activities), and a HDD up to 80GB (unless you have an insane amount of files). The average person doesn't need a super high-end GPU, 4 Gigs of DDR2 1066, and two 500GB HDD in a Raid setup. Do research, shop around, compare prices, and stay within a certain price range - $500 will make a self-built system for a few years to come and $800 will buy a system for a few years to come.