if you already have a 560ti, you should get another. If you plan on gaming at over 1080p resolution you may want to opt for something with 1.5-2gb VRAM.
If u need to get a new motherboard just to SLI, I would recommend to get a GTX 580 and sell ur GTX 560Ti, it will deliver less performance (in highly optimized SLI games only), but will prevent flickering and microstuttering
I think a 560 Ti SLi is a decent option over all, but make sure your heat disipation is up to the task as 2 gpus tend to heat up quite a lot. Also make sure your PSu will be able to handle the extra power required and have the necesary cable connectors.
Right now nvidia is sort of changhing its 560 Ti line up to a diffrent core (im guessing it wont be compatible with older 560 Ti for SLI) so you might either wait for price drops, or buy fast afraid that the old 560 Ti wont be available for long.
Another option would be to wait with the upgrade untill the next generation comes out (ATI 7000 and nVidia 600). Thats what im doing in the same situation as you .
I have a couple of 560ti's in SLI, see my sig, and I have never noticed the "micro-stuttering" problem I hear so much about. If it is there, it so small it is unnoticeable, to me anyhow.
What I still struggle to understand is why, especially when value for money is noted by the OP, people still recommend a 580?
560 SLI has a slight performance edge over the 580 and will cost you 50% less. Although temps will be roughly 15% higher and power consumption about 20W more.
If u need to get a new motherboard just to SLI, I would recommend to get a GTX 580 and sell ur GTX 560Ti, it will deliver less performance (in highly optimized SLI games only), but will prevent flickering and microstuttering
I don't see any flickering or micro stuttering with GTX560Ti's in SLi and yes, I am running such a setup so I must have as much credibility as you when you tell someone that SLi'd GTX460's get as hot as hell.