Well what you mentioned your primary uses you really don't need that powerful of a computer. Your internet speed will be dictated by your ISP, photo editing might be the biggest toll on your system if you did SERIOUS editing, but if it's making corals look pretty then you won't need the muscle. Your video needs most likely will be CPU limited more so than video card based, as even most onboard video are quite adequate to deal with the streaming needs.
My advice to someone such as yourself, look on dell.com, get something with the Core2Duo chipset, Quad Core is a waste IMO, you can pile in as much hard drive space and memory as you want through their customization menus. Even going without a monitor if you already have a LCD that still has some life left in it. If you don't absolutely need the machine now, I'd say wait as Windows 7 is supposed to come out next month IIRC, and you'll be paying for the operating system whether you want to or not, not sure if they allow you to strip it from the machine, so your O/S for your last computer could still be used if you don't like Win7.
The Inspiron or Vostro (small business) probably would be your most economical bets. Either way you go I'd go for the 64 bit operating system just so you have the ability to go beyond 4 gigs of ram in the future. If you want massive storage solutions, I'd probably just buy a 2nd drive off newegg or something, but Its been a while since I've priced hardware so not sure if Dell is that much more for the upgrades in size.