it will work, but be aware that both will be working at the smaller memory, so both will be at 1.5gb that is ok for low resolutions but if you are playing with multimonitor 1.5gb may not be enough.
also their clocks are slightly different it is recommended to synchronize both, i think that the Matrix can handle a slight overclock to match the 580.
Basically the performance will be awesome, but for multimonitor systems you may be bottlenecked because of the 1.5gb so in some situations the single 580 classified at 3gb can perform better because of the VRAM that is the major problem other than that the performance will be awesome.
ahh ok cool ty i don't use multiply screens but the matrix 580 is bloody big and to find another one that's cheep that's not over priced as now there kinder hard to find.... as well as the classified one... but found a cheep one ..and well will fit in with out airflow problems .. if i had got another matrix might have airflow and and its big ass is quite grr lol
question so the 3 gig will act like 1.5gig or will it still use it... i know about to synchronize both but still rather confused ..and will a 1000 watt handle it?
yes only 1.5gb will be used if the second card is only 1.5gb. to synchronize them you can use programs like MSI AFTERBURNER that have a syncronize both cards that will do. yes a 1000 watts psu can handle it probably, to make sure check if your psu is listed on the Nvidia SLI certified PSUS list: http://www.geforce.com/Active/en_US/en_US/pdf/PSU.pdf