I have a similar setup:
Intel Core 2 Duo CPU E8400
6GB RAM
Geforce GTX 460 1GB
I hope I can run it on high settings (@ 1680 x 1050).
But you should buy a GTX 460. Its faster and cheaper than a GTX550 Ti. The 550 Ti has alot VRAM, but the memory is slow. With a bit overclocking you can get a GTX 460 close to the GTX 470.
The 550 Ti is usually quite a bit less than the GTX 460 if you shop around. The HD 6790 however is the most powerful card I've found for ~$110 after rebates. I did find one GTX 460 at $120 after rebates though (which is way lower than every other one I've seen), which I'll link after the next response. Also, don't underestimate your card, which is significantly more powerful than what they recommend for high.
So your saying thishttp://www.amazon.com/dp/B003ZMTPEW/ref=asc_df_B003ZMTPEW1768845?smid=ATVPDKIKX0DER&tag=pg-469-34-20&linkCode=asn&creative=395097&creativeASIN=B003ZMTPEW is a good buy? (darn good price after rebate too.)
That's not a great price. It's more a typical price after rebate for the GTX 460. This is the best deal I've found thus far, but I didn't look up how people trust the brand yet:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814500202
Since the rebate is valid to 11/30 personally I'm cruising around for better deals though, and I'll grab it before the offer is over if I don't find better. I'll play it on a GTS 450 until then.