Keep in mind what you are using the computer for .
If you want a quieter and cooler machine then consider an e8400 or similar . Not that many people need quad cores . Most games run better on a C2D with higher clock rates
If you have apps and games that will use 4 cores its worth considering 45nm chips . Only diehard overclockers get good value out of the q6600
Also keep in mind monitor size . The 4850 is good for almost every game on a 22 inch lcd . If you have a smaller screen and dont plan on getting a larger one then an 9800gt or 4830 could be better choices . If you plan on a 24 inch monitor then maybe a 4870 would be a better choice
Honestly I wasn't too sure about the dual vs. quad but it seems to me now that dual would be the better choice as I honestly don't know off any apps or games that would take advantage of the quad core. The E8400 seems to be the better choice now.
Anyway, right now I have a 19 inch lcd and plan to get a bigger one soon but I don't think i'd get a 24 probably a 22 inch. Thanks guys.