Notice that the EA430 has only one PCIe plug. The GTX560TI needs two. They must have used a Molex to PCIe adapter to get it to work. And for me, a new machine shouldn't need any adapters.
You're pretty close to the limts of what your PSU can do. I don't think a hdd will put you over the edge, but you are much to close to the limit for a new machine. I'd take it back and see what they will do about getting a better PSU in there. Depending on the CM 550W PSU they were going to use the Antec might be better. But at this point I wouldn't go adding much more to that machine.
Itll work. On basic budget builds, an Antec EA 430D can power most upper middle gpus with no problem. I use to help friends replace old OEM psus with EA430Ds and midrange cards like GTX 460s / 6870s etc and they ran stable.
Antec is better. Any psu line from CM that's not the Silent M pro is utter crap (Real Power / Extreme power etc).
It'll power this budget build just fine and with no problems.
Since there will be no overclock possible with that cpu... we can do an easy worst case estimate.
CPU - ~80 watts.
GPU ~180 watts
Mobo ~several watts
Ram ~several watts
hard drisk ~several watts
Case fans ~ several watts
dvd drive and hard drive ~ several watts.
so somewhere between 240-280watts under the worst case. Well below the ~350 watt max for the 12v rail of that PSU. Standard usage will be well in the 130-180w range so you're absolutely fine.