Check out my build guide.http://www.squidoo.com/electronicandmore
The 550 Ti is not a really good card at all, better off getting the 6850 or saving and getting the 460 768mb. It'll defiantly be faster or save you some money. I suggest the 5850 which would be much better however availability of the 5850 is scarce so if you can wait for the shipping I suggest getting it off my link.
And with the $50 to spare in my build guide, use that to get the i5 2400, it maybe and extra 10, but I strongly suggest getting it.