I just want to stress that 5 years is really a long time especially for the demands of video and audio editing. Will the computer still be very usuable in 5 years? Of course, but it might be 1/4 the performance of medium priced computers in 2012. Often times the best route for price/performance over that much time is buying new equipment. Like instead of $1500 now for 5 years, a $750 system now and a new $750 system 2.5 years from now will give you better performance. Often times you hit a performance plateau where you get only get a marginal difference in performance for significantly more money. That top 10% in performance can cost you 200% cost.
The biggest points of investment for audio/video editing is memory and hard drive. You want to have enough ram to be able to hold your cache and undo's, which can be significant. For example, if you have enough ram to hold your undo for a 600mb audo file, that simple undo command might take 10 seconds. If you don't have enough space it will be cached in the HD and that single undo might take 40 seconds. This also means you want a fast, reliable HD. HD technology hasnt progressed that much in the last 5 years. Not like CPU, RAM, Video cards, etc. You really have to weigh the benefit to cost ratio's. Is a RAID 0 Raptor setup worth the possible 20% more speed then a 7200rpm drive set for $200 difference in cost? Is a RAID 0 possible 20-30% transfer speed increase worth a higher risk of drive failure and loss of data? Would making it a RAID 5 safety net worth $150-250?
These questions are answered by the usage requirements and personal opinion.
If it was me, I'd try to get a system around $1000 saving $500 for future upgrades. $1000 can get a very decent system for those needs. The one thing that can throw your budget is the RAM. Again as mentioned before, more RAM (and like jim1976 said, in quantity not necessarily quality) is important. 4gigs of higher CL, lower speed RAM can be much more valuable in this case then 2 gigs of high performance RAM for the same price range. Storage is also a large concern. 500gb might get used up in a video editing environment very quickly, especially if there isnt a good archiving strategy. Also, planning the usage can be important. I keep my system/application data on a seperate drive then my video/audio storage, and a seperate drive/partition for my cache/temp working storage. Cache/temp work storage should be on the fastest drive(s) and due to high fragmentation, should be put on a seperate drive/partition structure. Another point is that if you are doing alot of processing/rendering of video/audio work, it will really give your CPU/RAM a work out. High CPU speed and in this case high speed RAM is important.
There's alot to think about and depends on the needs. This is also why I would setup a system around $1000 on a very scalable setup leaving a $500 budget for upgrades. See where your deficiencies are and move from there. Make sure you pick a good case considering that you might want alot of drives in there with cooling enough to handle the excessive heat, solid motherboard for scalability, solid CPU. Ram quantity, HD quantity is easily scalable. Your case/PSU cost is way too overbudget for that price range IMO. Almost $300 for case and PSU for a $1500 system is a bit much. I'd try to keep it around $150 as that is a very reasonable range to get something effective for your needs. You could also cut costs on the 6600 processor to the 6400 for a 5-10% difference in performance for a $90 price difference. Also that motherboard is a bit much too IMO. Could save $100 there too.
I'd recommend cutting costs where you can to reduce the initial cost and leave some room for upgrading.