Another bang for the buck?

My main usage is no longer gaming since I don't have the time. What I find myself doing mostly is using SolidWorks o9 + or PhotoShop + web surfing on a second monitor while listing to music or streaming NPR. I also have my HDtv connected to watch hd MX races that currently play in slow motion.

My current rig is;
athlon xp 2500 - Asus A78N delux - 2gb ram - All in Wonder Radeon 9800 - Klipsch 5.1. its been around for quite a while, and still working great except for laggy video and skippy music when multitasking. When I say I get my moneys worth, well you get the idea.

Anyways since my dads computer just died (not worth fixing) I figured I would just give him this one and build another. My goal is to spend the least $ and still be able to multi-task and not have laggy music. and watch the latest video. Thanks for your help!

Do you keep the case, PSU, DVD, HDD ?

I guess some of the questions I have are, do I need quad core processing or will dual core fit the bill for what I'm doing? Is DDR3 that much better then DDR2? and where I really need help is matching the best Mobo to the processor. I'm hoping since this isn't the latest greatest build some of you should have the bugs already worked out?

here is my current rig