Upgrading my Gaming/Video Editing Computer... Please Help!

I'm currently looking to upgrade my Alienware by outfitting it with a new motherboard, RAM, Processer, and Video Card (No need in letting my awesome tower go to waste). I want to keep everything to around $500 so this is what I've been leaning on. I use my computer primarily for video editing, sound editing, and gaming.

-MSI K9A2 Platinum Motherboard (http://www.newegg.com/Product/Prod [...] 6813130136)
-AMD 64 X2 6000+ 3GHz dual core (http://www.newegg.com/Product/Prod [...] 6819103773)
-OCZ Platinum 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 1066 (http://www.newegg.com/Product/Prod [...] 6820227298)
-SAPPHIRE 100242L Radeon HD 4850 512MB 256-bit GDDR3 (http://www.newegg.com/Product/Prod [...] 6814102747)

I also hear that the HD 4850 runs very hot, so I'm not sure if it's necessary to buy a heat sink as well. If so, which one should I look at? And do I need additional cooling for my processor if I don't plan on overclocking?

Additionally, I currently have a 450 watt power supply and I'm not sure if that will be sufficient for the new upgrades. Will I be needing to upgrade that too?

Thanks much for the advice,
Erik S.