Your video/sound/picture editing will all hugely benefit from the extra cores.
If you have it in your budget, get a i7 with 12GB of RAM.
I don't know if either 4GB or 8GB would be better for a C2Q however.
It appears that AutoCad 08 doesn't require a large amount of RAM, so 4GB seems perfect.
However, CS4 will gobble up whatever you throw at it. So if you can get a 8BG (for C2Q, either 4x2GB or 2x4GB) or 12GB (for i7, 6x2GB [6gb sets only cost $100 anyways]) then go right for it.
What processor you have doesn't really matter for music/video watching however. Audio is fairly simple, and video relies on the GPU primarily.
Your heavy multitasking programs will gobble up all your cores.
2 cores ar 3.6GHz is essentially 7.2GHz of power. 4 cores at 2.6GHz is essentially 10.4GHz of power (not to mention the i7 has increased clock efficiency over the C2Q, placing it even more so).
A dual core with 4GB with be the cheapest, however going for a i7 and 12GB is the pimp way to go.
HOWEVER!!!
If you really want to go crazy (or me, because I'm bored), AMDs sexa core CPU (6 cores at 2GHz+) cost around $400 each. Tyan makes a quad socket mobo which supports a quad socket expansion card. The total system supports 8 CPUs and 128GB of ram. The expansion board and the motherboard could be gotten for $500-800 (if I remember correctly).
This is a combined 105.6GHz (2.2GHz x 6core x 8CPUs = 105.6GHz theoritical) power ($450 per 2.2GHz sexa core, coming in at $3,600) , with 128GB of RAM (using 32x 4GB DDR2 800MHz modules, $150 per module x32 = 4800) for another $4800.
I can't imagine what kind of PSU you would need for this, so I'll assume $500.
Now imagine, all that for only $9,500! All in one system! Would only take a high power AC to keep it reasonably cool!
($600 for mobo/expansion card, 500 for PSU, 4800 for RAM, 3600 for CPUs)
Who needs a GPU for video editing when you have that bad baby?