How can I replace video cards on a motherboard that supports DDR3 technology?

[FlyingBlind]

I feel ridiculous because since I've decided to go solo and work part time in order to get my own business off the ground - my computer failed. More accurately my two Nvidia GeForce 8800 GTX cards failed. I got them brand spanking new when they came out and they've died this week so I'd really appreciate any feedback at all. Is there any way to replace them with newer cards and not be forced to do the domino upgrade? (You have to buy a new motherboard to support new vsfx cards, then new CPU for the motherboard, etc.)
This is my current build since I've only been able to offer help if I know as much as I can.

Windows 7 x64 OS
MSI P7N SLI Motherboard
2 Nvidia GTX 8800 video cards via SLI
4GB RAM (DDR3)
CPU Intel Quad Core Duo
Cool Master Stacker full tower
1KW power supply

My budget is $400 - $500 max so I'm just trying to see if there's a way to get a video card or two similar to the ones I had before using my current motherboard. Everything I see on NewEgg, Tiger Direct and B&H are DDR5 with no legacy or refurbished parts or older bare bone kits with DDR3 graphics cards in the $500 +. I'm not keen on buying refurbished anything especially with tech. I'm not sure if it's backwards compatible but any advice on this would be great. Right now I really can't afford to upgrade and desperately need some advice. I'll repay any way I can. I do know a lot about programs so if you have questions on Maya (primarily 2011), Mudbox, Photoshop (switching to Gimp), Illustrator, some After Effects (5 version).

 

[Wolfshadw]

Your Motherboard RAM and graphic card RAM have nothing to do with each other. You can use a GDDR5 graphics card in a DDR3 motherboard without issue. I'd probably be looking at something like this ASUS Geforce GTX770

-Wolf sends

 

[nostall]

Roger Wolfshadw. Just yank the 8800's (mine died too awhile back) and stick a 770 or 760 in there. I think you'll be happy.
You have plenty of power, so go for it.