I remember building my current computer at the end of 2002 with a P4 2.8ghz cpu that cost $450

. I was young and blew all my (parents) money building two computers that cost a little over $3000 in total

.
My budget now is around ~$500.
I have a pretty good idea for what I want but I am really struggling as for which processor I want to get. These are the ones I narrowed it down to:
Core 2 Duo e8400 3.0ghz 6mb cache 45nm
Core 2 Quad q9450 2.66ghz 12mb cache 45nm
For as long as I remember I wanted to get the e8400 since its a great price for a 3ghz chip and it runs cool and uses very little power. I never wanted to get a quad core because I want to minimize heat output and power use but with the above prices its hard to not spend $60 more for double the cache and double the cores.
So people know, I don't play games but I like my computer to be future proof since my current one lasted me almost 6 years I don't upgrade much.
Which is it then? Dual core or Quad core?
the rest is probably going to be a nvidia 9600pro, 4gb memory and some p35 motherboard. I will use my current drives/case/powersupply (480watt).