It depends on your resolution and what you demand out of games. If you're running 1920x1080 or higher, than it'd be a good idea to SLI if you want smooth framerates and eyecandy. But if you're running a lower resolution, I'd vouch that you reconsider purchasing a second card.
Make sure your power supply is capable. I have two 470's in SLI, and they can make my 750w Corsair PSU beg for mercy when the cards are OC'd and stressed. You'll want a good 650w unit at the least.
The heat generated by my 470's is also considerable. The 465 runs cooler by comparison, but you will want to make sure the airflow in your case is ample. Also, check that the spacing between the PCIe x16 slots is ample. I have two inches of space between my cards, and even though they're in an HAF 922, things can get toasty.
The extra heat that comes with two cards also means more noise. Your fans will be working harder, and so you'll have to ask yourself whether you can deal with a louder PC. You can use a program like MSI Afterburner to create fan profiles if you haven't already; I've found it incredibly useful.
If you can deal with more noise, a warmer PC, and your gaming needs justify a better framerate or eyecandy, then go for it.