With those prefered parts i'm not sure it is posible to get a build for 800$ or less the CPU, GPU and case is 506$ atleast, for that GPU you'll need a rather powerfull PSU and i would recomend a AMD870 chipset motherboard those two you should expect to be over 250$ giving you 45$ left for a harddisk, memory and an ODD that just won't do. If you are just running 1360x768 ( by the way 720p is 1080x720 just that excat resolution not higher that something different ) the 5850 seem a bit overkill really a 5770 or gtx460 768mb will do just fine, and make it more likely to find something for 800$ or less (if the parts have to be from the site you linked that is). I see no posible way of getting a HD 5850 with your budget sorry
here is what i suggest(keeping your prefered CPU and case)
I don't know the firm who have released that version of the GPU, and i wasn't able to find any reviews by doing a quick search so i can't really recomend it, but it was the only GTX 460 available within your budget and i would expect it to do just fine. If you are more of and AMD/ATI GPU fan you can go with an HD 5770 posible this one: http://www.mwave.com.au/sku-42060468-HIS_Radeon_HD_5770...
If the ODD you've got can play dvds you should be fine, i'm guessing we are still a few years away from blu-ray or something like that beeing the standard for computer games, thats like 20$ or so saved there but as i said i would suggest you tage a GTX 460 instead of HD 5770, i did find a review of the GTX 460 card but the HD 5770 is on the charts it might be of use to you: http://www.hardwarecanucks.com/forum/hardware-canucks-r...
I know they are both more in my price range but there is something about the 5850 that seams so appealing all the benchmarks of it winning by about 10fps i really gotta snap out of it, lol
well i guess it will all depend on the price they are when i buy them.. with anyluck it will drop $100 XD (in my dreams)
anyways thanks, you have been tons of help wasn't even considering a Nvidia card lol