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5770 will be a downgrade.

Your cards are good enough for anything upto 1920X1080

Two 5770's running in CrossFire will be about 20% faster than your current GTS 250 SLI setup.
A single 5770 will be significantly slower.

Stick with what you have, it is still quite good.

A single 5770 is a downgrade. 2 x 5770 in Crossfire is nearly a lateral move - slight advantage to 2 x 5770 over 2 x GTS250.

Your setup should run games very well. If you feel you truly need more performance, you might get it with a single 5870 or GTX480, but even they might not give very much improvement over your current setup.

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