O SLI não adiciona mais memória à configuração existente. Tudo o que ele faz é aumentar o poder de computação usando a ponte SLI. Portanto, nada está errado quando diz que 1GB ainda está no seu sistema. A razão para isso é que o link SLI não pode transferir dados com rapidez suficiente sobre a ponte e, portanto, deve abandonar um dos bancos de memória da placa.
Seja como for, você deve atualizar seus drivers o tempo todo e prestar muita atenção quando novos jogos forem lançados.
Vou citar algumas FAQs de um tópico freqüentado em overclock.net, para responder às suas principais preocupações. O tópico está localizado aqui
What is Nvidia SLi technology? SLi technology allows the use of multiple Nvidia graphics cards in a single computer system with a motherboard equipped with an nForce SLi media and communications processor (MCP).Do you need to have dedicated hardware in order to run SLi? Yes. You will need an nForce based motherboard with two PCI-Express 16x graphics ports and two matching GeForce graphics cards which are “SLi Ready”. Alternatively, if you have an Intel Skulltrail motherboard, or any motherboard using an Intel X58 chipset, you can also run SLi.
Is it possible to run SLi on a non-nForce based motherboard? It ispossible, but will only work with older SLi capable cards. You would have to make sure that the motherboard has two PCI-Express 8x slots, so most Intel 965 and 975 based boards will work. You would also have to obtain the hacked 85.96 drivers, which means that you won't be able to run any newer cards in SLi. At the moment you cannot run "hacked SLi" on any cards that came out after the 7900 series, so that relates to the 7950 cards and the entire 8 series that was released after. With this in mind, and the fact that it isn't offically supported by Nvidia, it's not recommended to go for a setup like this.
What graphics cards support SLi technology? Most of Nvidia’s PCI-Express graphics cards starting from the 6600 GPU can be run in SLi. However, it is always best to check before you buy that your graphics card is "SLi Ready". You can find the current list of Nvidia GPU's that support SLi here.
When two cards are run in SLi does the memory count as being doubled? No, you do not add the memory of two cards in SLi together as both cards store the same data and textures that are being rendered simultaneously.
Can I mix and match graphics cards with different GPU’s? No. Both cards that are to be run in SLi must have matching GPU’s. For example, a GTX480 cannot be paired with a GTX580.
Can I mix and match graphics cards from different manufacturers? Yes. The latest ForceWare drivers allow pairing of cards from different manufacturers. For example, you can safely pair an XFX 8600GTS with an EVGA 8600GTS.
Can I mix and match graphics cards if one of them is overclocked by the manufacturer or has been volt modded to attain higher clocks? Yes. When using two graphics cards with differing default core and memory clocks, the faster of the two cards will lower its clocks to match the speeds of the lower card.
When I overclock in SLi will it apply to both cards? Yes. Though you will only see one slider bar for the core and one slider bar for the memory, any alterations you make to the clock speeds of core and memory will be applied to both cards simultaneously.
Can I mix and match graphics cards with different sizes of memory? This is not recommended, so it would be better if you could consider the answer as a “no”, but it is possible using CoolBits. Though at the end of the day, if you currently own an 8800GTS 640MB you should purchase a second 8800GTS 640MB. However, when using CoolBits (value set to 18), you can force both of the cards to use the lower of the two memory sizes and operate together in SLi mode. When dissimilar memory sizes are enabled to work together the effective memory size for each card becomes the smaller of the two memory sizes. So an 8800GTS 640MB paired with an 8800GTS 320MB would operate the same as 8800GTS 320MB SLi.
Can SLI technology work with AGP or PCI cards? No. SLi technology is exclusive to the PCI Express bus due to its superior bandwidth and support for isochronous data transportation.
Can I run multiple monitors when using SLi? Yes, but you will need the latest Forceware drivers, available from nvidia's homepage. Only drivers from 180.48 onwards will allow you to run multiple monitors when SLi is enabled. IF you don't have the latest drivers you'll have to disable SLi in nVidia control panel in order to get multiple monitors to work.
What operating systems are supported with SLi technology? SLi technology supports 32-bit (x86) and 64-bit (x64) Windows XP, Vista, and Windows 7, as well as 32-bit and 64-bit Linux (IA-32 and AMD-64/EM64T).
I have three of these cards, only two are in SLI, can I use the third one for PhysX?
No.It seems that when the drivers detect three identical cards (even if they aren't Tri-SLI capable), it doesn't let you use the third card only for PhysX because they figure what is the point if you'd rather do Tri-SLI and GPU PhysX. This applies wither or not your cards are Tri-SLI capable or not. It is possible that a driver bug causes this. You'll need a different card to have dedicated PhysX support.
Espero que isso elimine suas dúvidas.