You're right, infact this forum has needed that said in a long while! So congrats for that. Multitasking is truly subjective. For you, running AIM, a spreadsheet, maybe some songs and what not on a PIII is perfectly fine. But unless you're one of those guys that runs like 60+ programs then an X2 is NOT for you. Get an older P4 with HT technology. They're not that expensive and really do help out. I had a 600MHz Pentium III a few months back before I upgraded to a Pentium 4 at 2.8GHz with HT technology. It runs a lot of programs really well. It's great for when you want to install something quick, say you just downloaded a cool media player and have to install it, on a typical Pentium III this can take some time, but with somethign that has HT technology it goes a lot quicker, and allows you to run multiple programs simultaniously. For instance say you're installing the program, but still want to listen to music and surf the web, on a Pentium III your install will take a lot longer and your music may end up lagging, I knwo this because I have done it.
But with somethign that has HT technology you'll experience very minimal lag or no lag at all.
So for you a high end CPU is NOT for you. Get something that suites your needs. A Dell is definetly for you, although more then half the people in here will disagree and say Dells are the devil, but don't pay attention to them. I'm sure you can find a decent PC that has pentium 4 w/ HT for a good price, say around $400, which was the last ad I saw for a Dell, so that could either be higher or lower.
But yes, don't get an X2 unless you plan on running MP3s, a video editing program, surfing the net, installing multiple programs, chatting on multiple messengers, while all at the same time copying and deleting files from one drive to another.
So I hope this gives you another perspective on multitasking and I hope it helps you in your quest to get another CPU when the time comes.