Sorry to hear about your computer. There's one thing worth looking into before you buy a new system. What exactly is the problem? Does it turn on at all when you hit power? If not, it could just be the power supply. I find that a computer power supply is almost always first to go. Replacements go for around $75-200, depending on quality and wattage required. If you're still happy with that system, it might be worth checking out.
Of course, this breakdown could be an easy excuse for an upgrade.
Regarding the two cheapest options dual and quad-core CPUs, here's a generalized speed test.
Dual E8400: http://www.cpubenchmark.net/cpu_lookup.php?cpu=Intel+Core2+Duo+E8400+%40+3.00GHz
Quad Q8300: http://www.cpubenchmark.net/cpu_lookup.php?cpu=Intel+Core2+Quad+Q8300+%40+2.50GHz
Is that speed bump worth $70? I think so.
While the quality of Dell is fine, I'm always leery of purchasing computers from big names like Dell. They'll ding you on the extras. For example, another extra 1TB drive is an extra $400?! I'd suggest purchasing a bare-bones system from Dell, and buying any extra components from either a local store or a site like newegg.com.
Personally, I prefer to support local "computer" stores, since the guys running those places KNOW computers and are fairly competitive when it comes to prices. You can walk in and build a system to match exactly your needs... get the best bang for your buck. The other reason is if something breaks down, I can deal with that local store for service and support.
One last suggestion: Get at least 4 gigs of RAM.