Upgrading motherboard from Asus P5KC E3155

I have an Asus P5KC E3155 motherboard with quad processor, but didn't know at the time of my purchase a few years back that though advertised as having an 8 gig RAM capacity it can only actually take up to 4 gigs of DDR3 though there are enough slots to handle up to 8 gigs of DDR2.

I use this desktop for work running several large Adobe products at the same time, Illustrator, Photoshop, Audio & Video editing software, but I'm now also wanting to get into some gaming in my leisure time and want to upgrade without it costing me an arm and a leg.

I need to upgrade my video card and would like some recommendations, but I would think that upgrading from 4 to 8 gigs of RAM might help as well since I'm running all those programs (if for nothing other than to have room to grow for the future).
The second factor is that I have the option to purchase 8 gigs of DDR2 and stick with my current motherboard and processor or upgrade my motherboard to something that can handle more than the 4 gigs of DDR3 RAM I have currently.

My budget for components (other than the graphics card) is $400 what would you recommend will give me the most bang for my buck using as many of my current components as possible but having to at least upgrade to a better graphics card than the cheapo that came with the system.