Radeon HD 7850: will my computer hold it and will it work?

If you have such an old pc, why buy a pci 3 compliant card... just get a 6870 or GTX 480 or 560. Or if you havent purchased the card yet, just buy a GT 640 which doesnt use psu power. if you arent that crazy about games, then just get the 640 and save up for a new pc.

The main problem I'm seeing is that any modern dual slot video card will cover at least 2 SATA ports and maybe 3 of them. Also, yor mobo probably has PCIe 1.1 which will work but severely limit bandwidth It's usually a mistake to try running a high end GPU on an older mobo; too many potential bottlenecks. I'd get the best single slot solution I could find, until upgrading the whole system is an option. Here's one http://www.newegg.com/Product/Product.aspx?Item=N82E16814202002 Good luck!