The core 2 quad Q8300 $149.99 at newegg. And the core 2 E6500 is only $79.99. For the price difference between the Quad core and dual core your not going to see an equal jump in performance on games at least. No the dual core isn't going to be better than the quad core in some things. If you had the dual core it would be good for games because of it's speed. The dual core has enough processing power to render what it needs to and enough speed to keep up with the demand on the mid rang video cards. On the other hand, if you get the quad core(Q8300) it isn't going to be as quick for games, in general, as the faster duo core, but will be faster at doing big tasks that utilize all four cores.(To my knowledge, I don't know of many games out there that actually benefit from four cores, maybe the new ones benefit, but it may not help all that much because of the Q8300 lower clock speed). The four cores would help with the burning of videos and other CPU heavy dependent tasks. If you are into overclocking things to get a little more performance that might change the look a little bit on the Quad core.
In all, if your on a budget but have ~$200 and want to do more coding, burning, etc., and don't want to do as much gaming than the four core could be a good option. If you want to have more gaming potential, than the $150 would get you more gaming performance, because you could also get a better video card around the ~$90 rang. What do you want more, gaming or power for general desk top programs? Also the dual core is still going to be a pretty big increase in power over the single core for coding, burning, and all that, as well. Here's the cpus at Newegg.
-Core 2 Quad Q8300
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115207
-Core 2 Duo E6500
http://www.newegg.com/Product/Product.aspx?Item=N82E16819116093&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-RSSDailyDeals-_-na-_-na&AID=10521304&PID=3463938&SID=4fqg6ccfqw
If you wanted to get the dual core than you could get a video card the would be 3x-4x as good as your current one. If you want to stay with Nvidia, I would suggest something like a 9800gt or 8800gt they are the same really, I've seen them go down to $60 new from Newegg.com. Or if you want an ATI card, the HD 5670 would be equally good but has Direct x11 and EyeInfinity, more future proof you could say, I've see it on sale for about $60 also.
-Maybe not this one but something like it. ASUS 9800GT 512mb(You need to have I think 26 amps on your 12v rail, or a psu bigger than ~450w)
http://www.newegg.com/Product/Product.aspx?Item=N82E16814121352R&cm_re=9800gt-_-14-121-352R-_-Product
-ATI Sapphire HD 5670 512mb(This one doesn't require any extra power from the psu)
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102870&cm_re=HD_5670-_-14-102-870-_-Product
Hope this helps
P.s. wow, I started writing this when there where only 5 posts.