Going from 1280x1024 to 1920x1280 is a big jump in resolution, you need a better graphics card. How much are you willing to spend on the upgrade? A motherboard upgrade will only get you so far as the IGP on the motherboard isnt used if you have a discrete card installed like you do. The difference between a HTPC and gaming is that a HTPC doesnt have to render each frame that it sends to the screen unlike in gaming.
Corsair pricing on newegg got screwed up a while ago so this PSU is currently a great deal
http://www.newegg.com/Product/Prod [...] 6817139005 Same price as their 550VX and you wont need to upgrade it even if you decide to do crossfire in the future.
As the 4850 isnt a ton more powerful than your 8800GT you may want to get that PSU while its on sale and save up for a bit and get a 4870 which will be a bit more suited for your resolution.