All the support I know of will not give support (*good support!) for mismatched cards even if they work. Since it's not well supported my advice is don't do this.
As to the power supply I use a few tools to figure this out. First I look at the Amperes on the RAILS and see if my cards will exceed some rail total current. Next I use a P3 Watt Meter on the system and see how many Watts the system uses while the game is running (that's a 20 buck thing!)
Armed with those numbers I can make a decision if a new power supply is needed. I never want to get close or over 70% of the rated power of the PSU. The P3 Watt meter is the input power so worst case is 80% efficiency so I pull back to 70 per cent.
Hope this helps.
Ok, what I would do? I'd head to http://www.tomshardware.com/reviews/best-graphics-card,2964.html and see how much card I can get for the money I allocated and then compare my old card to the new card.
Hello, I want to ask you couple of questions. I have GTS 450 2GB DDR3
and I want to put GTS 450 GDDR5 1GB, will it work as a SLI ? The second
question is if I put 2 GTS 450 (mine the 2GB edition and GDDR5 1GB
edition) without that SLI will the gaming performance improve or it will
remain the same ? The third question is Can I use different GPU as a
SLI example GTS 450 and GTX 460? The Fourth question is can I use 550W
PSU for those 2 GTS 450 ? Thanks in advance