Looking to upgrade, how can I tell how fast CPUs are? what to get?

Hi everyone - first off, apologies that I'm pretty ignorant about hardware.

I have a Dell Dimension 4700 that still runs pretty well, but I would like to upgrade. I'm having a really hard time deciphering what processors are best, and what would give me the best bang for the buck.

My current machine has 2GB of RAM and a Pentium 4 3.00 GHz processor. I guess I'm essentially trying to figure out what kind of a speed improvement I would get by upgrading.

My use/preferences:
- normal stuff (web-browsing, Excel, etc.)
- programming (nothing too intense)
- VPN/remote desktop into work
- a lot of streaming video watching. Firefox cannot keep up with streaming Netflix on my computer - I must use IE.
- video conversion (e.g. dvd to .avi format)
- I would possibly get into video editing
- no gaming whatsoever (other than little flash games)
- I'd like to get an HDTV and output video (from Netflix/Hulu/Boxee) to it. So I guess I would need HDMI out.
- it would be very nice to have a box that is very small and compact
- ideally around $400-$500, but can go a bit higher. Definitely nowhere near $1000 though (I don't think I would make use of anything that expensive, would I?)

One machine I was looking at is the Gateway SX2800 series:
- The SX2800-07 has a 2.5GHz Intel Core 2 Quad Q8300 Processor for about $520
- The SX2840-01 ($560) has a 2.93GHz Intel Core i3-530 Processor (dual core)

I really just have no clue what kind of increase in performance I'd be getting with either of these machines. I would have thought that a quad-core CPU would be much more expensive than a dual-core, but I guess not?

I guess my question is really two-fold:

1) How much faster are these (or comparable machines) to what I have now?
2) Any suggestions for a particular machine or at least what CPU to get? Even though I am comfortable taking the box apart, I'm not really looking to put something together myself (though I guess I would consider it if it's worth it).

Thanks in advance!

first : upgrading probalby isn't worth it. You won't be able to put much better of a cpu in that system.

second : the q8300 is going to be ok. You may be able to get a better cpu for less. I was looking at a dell poweredge t310 on dells site the other day... i think it had a 2.4ghz-2.66ghz xeon (i5 equiv) 2gb, 250gb, for $399.

I didn't order it because the hard drive was listed as $129, and they wouldn't sell it without the hard drive for $270