I currently have an ati 4870 512mb video card and it took a turn for the worse. I'm looking to buy a new one but not sure if my computer will handle something like a gtx580 or an ati 6970. I'm also looking to buy a complete new PC, just not at the time. (maybe a few months or longer) Also thinking of doing sli or crossfire with the new system. My thoughts was to buy a good video card and then just put it in the new PC when I'm able to upgrade to it
This is my current system
Motherboard Biostar TA790GX 128M
Processor AMD PHENOM II X4 940 3.0GHZ
Monitor LG FLATRON 23''
Primary Hard Drive 500GB SATA II 3.0 GB 16MB 7200RPM
Secondary Hard Drive WD MY BOOK ELITE 1TB
RAM 4 GB PC6400 DDR2 800
Graphics Card ATI RADEON 4870 PCI-E 512 DDR5
Power Supply 800 watt (make is cyber power)
Your computer will "handle" it just fine because of your PSU and motherboard. The big thing is that with a 580, and at a 23" monitor, your CPU will probably become a limiter, but not by much that you'd notice anything.
that psu you listed def sounds a bit sketch, get a quality psu, oc that cpu and you will be fine to run a 6970 or gtx 580, but by no means will you run a gtx 580 on some no name psu, not a smart idea. Also with your cpu at stock you will see a decent reduction in performance imo. make a couple small changes and upgrade your gpu and you should be able to get some more usage out of this current rig you have
If you're going to pour big money into your system, don't do it with junk for power. This is also a component you'll be able to take with you to your new PC. A Corsair 850W or higher is what I'd recommend for SLI. I say this because my system will reach to the 670W range when gaming with both 580s near 100% load. The Corsair 850W covers all the wattage you'll be using on the 12V and gives you a little cushion for whatever else you're doing: