You have some fundamental misunderstandings about graphics cards. Specifically, you seem to think that more memory makes a card faster or that all cards with the same amount of graphics memory are somehow comparable. In fact, more memory just makes a card capable of displaying more complex graphics but at a lower frame rate. (Assuming the bit width of the memory is the same.)
Also, a 512MB card may happen to be really, really slow and a 256MB card might be incredibly fast. It just doesn't tend to work that way among cards of the same generation because it's hard to sell mismatch products. (Honda doesn't make a minivan with a spoiler, but it's not because spoilers are expensive and can somehow only go on fast cars.)
Often manufacturers will try to distinguish themselves by putting lots of memory on a slow card. The net result is that their card looks better than the competitors. But in fact the memory is useless because making the card use that much memory would drop the frame rate in the toilet. (More memory just takes longer to access and process.)
In any event, your graphics card is a toy. Also, it's an ATI-based card, which doesn't do SLI. In any event, SLIing two crappy cards will just give you one big, crappy card. SLI and Crossfire only really make sense for very high-end cards where you the price/performance ratio drops off steeply. When you're that far below the bend in the P/P curve, SLI/Crossfire makes no sense.
And, SLI doesn't make your memory add up, except in marketing terms. If you SLI two 512MB video cards, although you will have 1GB of memory, it will be functionally the same as 512MB. (Because each video card will need its own copy of almost everything.)
In sum, get a graphics card that meets the minimum specifications, or ideally meets the recommended system specifications. The game will perform much better.
Seriously, would you please send me a message and tell me what card I should get next. I truly have no clue and I do not want to buy something junky. I currently have an Nvidia GTS 8800 640, I used to have two running in SLI, but one blew out. I would like to upgrade. Thanks in advance for any help you can provide.