Looking at benchmarks, it seems the GTS 250 OC 1GB edition is on par with a 4850 512meg non-OC edition...
Go for the Crossfire 4850's.
with that SLI board, you are kind of painted into a corner. i would search high and low for a deal on the 260's before i messed with the 250's though. they aren't bad cards but it is old tech that is rebranded with minor enhancements.