The GTX 550ti will only draw about 9.67A on the +12V rail when maxed out. http://www.hwcompare.com/category/gpu/
If your PSU can provide at least twice that on the +12V rail or rails combined, you should be OK. (The CPU and other devices likes fans, etc, also need +12V power from the PSU.)
Then, If the above holds true, use this power supply calculator to see if you have the necessary wattage:http://www.thermaltake.outervision.com/
Normally, a 300W PSU is not recommended for any card that needs a separate PCIe power connection, but I'm betting you will probably get by OK. If, during demanding
gaming moments, the machine shuts down or re-boots... it will be a sign the PSU is unable to maintain the necessary voltage levels. Then you'll know you need to upgrade.