SLI means that there are two graphics card (PCI-E) slots on your motherboard. So you can put in two graphics cards, connect them with a 'bridge' and get increased performance.
SLI is Nvidia only, the ATI equivalent is Crossfire
They have to be two of the same cards. You don't get "double" the performance, but you do get more.
If you want a significant increase in performance, I would suggest the following:
Nvidia 7900GS, which you can get for around $275 (
www.umart.net.au)
ATI X1950pro which you can get for around $245 (
www.fluidtek.com.au)
Or if you want to spend a bit more, an Nvidia 8800GTS 320Mb card would set you back about $390 (
www.fluidtek.com.au). This is DirectX 10, and would be a significant step up from the others mentioned.
(these shops are all in Sydney)
Unfortunately, as you have discovered, a 7300 is not a great gaming card. It is a step up from motherboard graphics, but is about the bottom of the line for current series graphics cards.
I don't think it would be worth getting another 7300 - better off going one of the above options.
A 7300 is not going to play a game like FEAR at full graphics options - I had a 7800GTX previously, and it would struggle with FEAR at full graphics. And that was a much better card than a 7300.
When you talk about a horizontal line going through the screen, do you mean like the screen is tearing in two? If this is the case try setting Vertical Sync to ON in the graphics options for the game.
Hope this all helps.