Don`t worry about the board too much!, what you want to be looking at is The PSU.
Do a Check of what the power requirements of the graphics card you are intending to buy.
Take note of how many Amps it needs from the 12v Pci-e power connectors.
No, a motherboard doesn't have a max wattage or amperage (I'm assuming that you're talking specifically about the PSU).
A PSU will only ever provide as much wattage/amperage as required, so it's basically impossible to "overload" a motherboard from a high wattage PSU. You could put a 1500W PSU with it and everything would be fine.
The 550 Ti won't affect anything like that either. The only issue there may be incompatibility because of differing PCIe versions, but that's unlikely.