2.5gb sli question

[jackcrow999]

hey jack back with another hopefully easy question regarding sli configuration,okay i have an evga gtx 570 with 2.5 gb vram okay i have been able to learn that by adding another 570 it will NOT double my available vram thats okay i read about how complex the problems with this is and that intel/amd have yet to find a way to pair mutliple gpu's vram together eficiently
so now here it is...my primary gpu is evga gtx570 w 2.5 gb can i add a 1.2gb 570 as secondary gpu as long as both gpu clock speeds freq's are equal will it downgrade to 1.2gb sli or can i maintain 2.5 available in sli or am i forced to buy another 570 w/2.5gb for second in sli config now because even though the extra $ for extra vRam is pointless @ this point (i have heard of some people mixing gpu's with mixed result and shaky driver support at best)i hope i havnt lost you in my NOOB thought wonder cloud thanks for any advice as im in the market for 2nd gpu for SLI in the next month or so just to help boost my fps for a little longer
my other NOOB question regarding dx11 what is a good dedicated physx card does it need to be dx11 for physx only? i dont know how much prosessing power or data is put for physx

[HiVizMan]

SLi mixed memory. Nope you are not going to get that one right. Or at least I have never been able to get it right. The cards are seen as being two different cards for some wierd reason. Or that is what I have had with the 2 different GTX580 cards I tried to work with.

PhysiX - The gains you will make by adding a dedicated physix card are going to be slight. If you have any nVid card then you are good to go.

[jackcrow999]

thank you for your reply i guess i will buy another 2.5gb card for sli and get mabe like a 9800 for physx or gtx550 idk i'll start with 2nd card first and worry with physx later sigh... but thank you again for your info seriously i did research all this and it seemed like every other post i read contradicted the last about sli and physix i understood what nvidia says is posible but i was hoping that there may have been a workaround for the vram issue as it really doesnt make sense except from a business standpoint to force you to buy the same card with exact same (useless vram) when all its using are the core/clock speed setting to (almost) double your through-put but thats my fault for buying the 2.5gb version thinking future proof! my mistake