Dual thread processors are still good for simple tasks, but I'll agree with JBI that they're now noticably slower than a quad with current software, unlike a few years ago when older software versions clearly weren't taking advantage of available cores/threads.
We have maybe 50 3Ghz dual-core Xeons left in the office and even the ancient 2.4GHz quads are starting to feel much quicker for basic things like booting up, word processing, internet and email.
If you're spending over $500, another 10% for something than can handle four threads is a worthwhile investment in my opinion.
