If you're not a gamer, save a lot of money on your graphics card by getting a HD3650.
I have read that video editing software may benefit from a quad core CPU. Check out a Q9450 or Q9550. Unlike the Q6600, they have the new SSE4 instructions that some graphics software may be able to use.
I agree with dirtmountain on the HDD. Depending on the criticality of your work, get two for RAID-1 or at least three for RAID-5.
+1 to the wd 640mb best price per gb you are going to find.
Dual core vs quadcore is sort of in limbo right now. if you were strictly gaming i would probably recommend the dual core as higher clock for the money. but with video editing and CAD you can take advantage of 4 cores. might be in your best interest to look into the q6600. you can even do a little overclock pretty easily if you need a little more speed from it.
ram - typically i recommend getting 1.8v ram. this is standard. anything with the voltage increased is typically to get the cl down. so sort of a company overclock. better to get it at its typical (jedec standard) and then oc if necessary. mushkin is great ram so might want to try and see what the price is for that same brand but 1.8 v (probably around $45 so save you a little)
great choices on board and gpu
system would be very upgradeable but as far as future use with newer programs, games etc you would probably want 4gb ram and vista 64 bit. but that is something you can do later when you have the money and you would only be out the $50 on ram you are spending right now.
You are well under budget. considering your usage i would look into a quadcore. your uses may take advantage and as far as future I would expect more and more programs are going to take advantage of multithreading. the q9550 is only $5 more than the Q9450. So basically if you go quad. I would pick between the 9550 and the q6600. Q9550 is best but most expensive. Q6600 is a good value and good overclocker. worth considering.
Going with a Q6600 has zero effect on the total cost, and even upgrading to the much better Q9550 still leaves you $225 under budget. Unless that $1000 budget was a worst-case-scenario, "I'll spend it but only under threat of death" kinda budget, I'd strongly suggest jumping up to the Q9550.
If you can wait a couple months, nahelam will be coming out soon and the holiday shopping season will also be starting. So if you're really hesitant to spend $325 on a processor, you'll probably be able to get it cheaper if you can wait a little bit. No guarantees, but it'd make sense that Intel would cut prices on the old generation of stuff when the new generation hits the market.