650W is plenty for your system, even with a second GTX 560 Ti and including some overclocking. This is based on this: http://www.guru3d.com/article/geforce-gtx-560-ti-sli-re...
Which shows 449W consumed at the wall, if the PSU was 90% efficient then only 45W would have been wasted as heat, which is a worst case for actual system power draw. Bear in mind that their test system was using an i7-965, which is more power hungry than any Sandy Bridge CPU and that 449W was achieved running furmark, which is usually much more stressful than any game, so your typical power consumption is likely to be much lower than 449W. But even so, 400W system power draw is only 61% of a 650W PSU's capacity. Personally I think a best case scenario would put your system closer to/under 50% utilisation of a 650W PSU.
If you think that 61% is too high, or if you know that you are going to be running furmark type loads on a regular basis, then you may be justified in choosing a 750W PSU, which is also likely to give you more upgrade options in the future.
Personally I wouldn't buy the Antec 300 because of the lack of cable management holes in the motherboard tray. I would check out the Cooler Master HAF 912 or Lian Li PC-K56 to see if you like those more than the 300, or if you like the Antec 300 enough to forego easier cable management.
The GTX 560 Ti is a really strong card for the price.
I agree on the PSU front, would also be handy for you to look at a modular one. Then you can keep the Antec 300 without worrying about cable management because the airflow and build quality is great in that series of cases. I have the 902 and my friend has the 300. (The 902 is worth a look if the budget allows. It's built like a tank, awesome case.)