I want to upgrade my graphics card to Sapphire AMD Radeon HD 6950 2 gb or to Sapphire AMD Radeon HD 6870 1 Gb card. Only thing i m confused about is will these card(any) going to fit in my computer case or not and also need some suggestion for PSU .
Odd case. While you might fit a 6950 in there it's going to intrude on the hard drive cage and you will need to remove the bottom hard drive so that you can connect the PCI-E power connector. You could go with a 6870 if the PCI-E connectors are rear facing so you can keep that second hard drive, but it's gonna be a pain fitting it in there.
With your PSU though I would go with a 6850 which is about as long as your current card
in my opinion you should go with hd 6870 which is good enough and will be good for 1080p resolution and fit in your case and overclock your cpu and gpu for more better performance that psu that you suggested will be good with two pcie connector because the hd 6870 needs 2 pcie connector so you will be fine with your upgrade and it will give you a performance boost.
yes upgrade ur cpu first then buy a gpu but not a hd 6850 from hd 4850 didnt worth it hd 6870 will be the best bang for buck and best performer of its price in the resolution of 1080p it performs quite well.