Hi,
First of all, you must check your mobo if it supports SLI. Not every i5/i7 mobo supports it.
Second - you must have appropriate PSU for it. Wattage is not everything you need, there's amperage for 12V lines, too - it must me sufficient, so look at label on your PSU and confront it with your graphic card's box.
And now - you can have some work to do, or just SLI
If you want just "plug and play"
so look for *exactly* the same card model - on Ebay, on clearance lines, newspaper ads etc.
This gives you 100% that SLI will work as you wish.
99,99% is the same core, but different model, brand, revision, or frequencies. Personally I was SLIing 2 different 7600GT and 8800GS - and it worked fine, adjusting faster card to slower's speed.
The special case is two revisions of the same cores (like GTX280 vs GTX 285) - I'm not sure if it works straight way, but it might work when you flash BIOS in one of them - drivers would see the same cores then, so SLI might be enabled.
The worst is different cores - some guys tried to SLI 7600GT and 7900GS - it worked, but flashing was needed. But I'm in doubt why to flash BIOS to make a card much weaker