Dual GTX 550 Ti's isn't what I'd recommend.
The GTX 550 Ti is a poor value to begin with. It's considerably slower than the GTX 560 SE and Radeon HD 7770 which only cost about $5 more. And the Radeon HD 6850 is even better for about $10 more.
Also, dual cards is a questionable approach to building a new rig. It's almost always better from a stability standpoint (along with power consumption, heat, noise etc) to go with a single higher-end card instead of an SLI or Crossfire pair. Performance in SLI/CF varies from game to game, and those setups encounter more driver bugs- occasional freezes, crashes, issues with sleep mode, etc. And dual-card setups are also subject to micro-stuttering. If you find the bang/buck of SLI irresistable, get a pair of GTX 560 SE cards instead (although you'd need a PSU with 4 connectors for that). But I recommend keeping that 2nd slot open to give yourself a cheap upgrade path later down the road.
http://www.tomshardware.co.uk/radeon-geforce-stutter-crossfire,review-32256.html
I'd take a single Radeon HD 7850 or GeForce GTX 570 for $250 over dual GTX 550 Ti's. Or Powercolor's Radeon HD 7870 for $280.
http://www.bit-tech.net/hardware/2012/03/05/amd-radeon-hd-7850-2gb/3
Also, that Thermaltake TR2 is a PSU to avoid. Corsair's CX500 and Antec's Neo Eco 520 are much better units for the same price. And I recommend spending a little more for a 600-620W PSU, you never want to be right on the minimum necessary to support your build. I'd go with something like these:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371031
http://www.newegg.com/Product/Product.aspx?Item=N82E16817256071
The Silverstone even has enough connectors to support a pair of GTX 560 SE cards.
But even without any of the suggested changes, your build would easily cruise through Minecraft and MW3 on ultra. For BF3 or Metro 2033 on ultra at 1080p, you'll want a 2GB graphics card (SLI and CF don't double your VRAM)