Is this a good power supply Brand and Model for a Medium Budget Gaming Computer and Why.?

The max power draw for the 550Ti is 116W so the 550W PSU will work just fine for you even when you upgrade to a better GPU later. New cards are requiring less power than the old ones do. Unless your planing on doing SLI or CrossFire you don't need all that power any more.

Your PSU is one of the most important parts in your PC. It is good your going with a good brand name unit. OCZ makes great SSDs. I have had 3 of them in my PC over the past 10 years. But they are not known for the best PSUs.

SeaSonic has been rated a leader in PSUs for over 10 years running. I have had 2 of them in my PCs for about 9 years now. They never over or underpower my units and they last forever. You really do get peace of mind with them. Buy a SeaSonic and you'll be very happy you did.

550w will be plenty for your system.

Personally, I recommend XFX Pro 550w 80+ Bronze PSU. The XFX Pro series are my absolute favourite PSU's I've worked with, they feel much better built, and give me better performance than any OCZ or Antec PSU i've worked with.

the 550w is very good, reliable, and will hold up well. all 3 of these power supplies would be good for most gaming.

It should work alright, but if you want the most reliable PSU brand you want to go with a Seasonic or Corsair.