Hi Span_Valalo,
The problem with Sli is that if you have 2 graphics cards with 896mb each of Vram the amount of ram that is available is still only 896mb, not 2 x 896mb. What does double is your "Cuda" cores, you will have 480 with your GTX275's instead of 240 single card. You might find that the GTX465 1045mb card will give you better performance at the moment on it's own. That being a GDDR5 as GDDR3 of the GTX275. If you haven't already got EVGA "Precision Tool" go to the EVGA site and download it (it is a free tool). See link: -
http://www.evga.com/precision/
If you have trouble getting it, send me a pm and I will send it to you it's about a 6mb file.
Cheers
Rick