Is it worth it for me to get a Quad-core over a Dual Core?

Okay so I've spent days researching the topic and still aren't completely sure. I'm basically building a relatively mid-budget Home/Office PC. I don't care about gaming performance but am primarily focused on Multi-tasking. I use my computer primarily for the following things, and usually all at the same time:

-Torrent downloading (always happening)
- P2P downloading (I try to stay away but I still use it fairly often)
- Web browsing inc. watching a lot of streaming video (usually have a ton of tabs open as well)
- Music playing (using media monkey right now but would like to be able to run iTunes if I need to)
- Document creation (standard MS office stuff)
- Virus protection and other small background programs
- Occasionaly I'll use photo/video editing software but it's pretty far and few between.

I would think that a quad core would be better for doing all of these things at once despite the fact that none of these programs multi-thread as far as I know. Simply for the fact that I don't believe most of these programs other than the photo-editing software require mass amounts of processing power. In other words the OS would run the multiple programs through different cores keeping them from essentially being cramed through the 2 cores of a duo. However I'm not sure if my reasoning is completely sound here since I'm still pretty new to this.

Anyway. My basic question is based on my use should I go for a quad core processor with lower clock rates or a dual core processor with higher clock rates. Two in particular I've been considering are the AMD Athlon II X4 630 (don't think I need the 3rd cache of the phenom since I won't be gaming) and the AMD Phenom II X2 550.

Any help or suggestions are appreciated.

almost worth it just for the photo/video editing as thos should be very multi threaded (ie could use the whole quad core)

if you are considering the 630, just get the 620, even if you OC there is almost no difference

and btw using a torrent is a kind of P2P file sharing system

Btw i was referring to mp3rocket/limewire etc. Don't know what that's considered

I'm starting with my CPU and going from there. Figured I would pick a CPU then the motherboard and then my ram, hard drives, etc.

I've been focusing most of my research on the CPU latley. My next big thing is going to be my chipset. Especially since I don't think I will be buying an external graphics crad since I'm not doing any gaming.

Go with the x4 620 I upgraded from a e6300 and never looked back no more slow downs when the virus scan kicks in while gaming or converting video