From retail shops, Intel 330. Or if you need/want 256GB, Crucial M4.
One 520 240GB. But I'd rather stick with the much cheaper Crucial M4 256GB. In either case, upgrading from a single drive is simpler than from a RAID or you could later buy a second 520 240GB and make a 480GB RAID-0. And again, expect SSD prices to depreciate quickly, as they have since the beginning.

In performance yes, but the odds are that you are unlikely to actually benefit from it, unless you have specific use cases that do benefit from the huge I/O throughput available. If you do not have such a use case, then going with single SSDs would be better over a RAID-0 array. Less cables, less power consumption, better air flow, lower risk of failure, simpler set up.
Please use the ATTO benchmark before following soultaker's advice. The benchmark results there should be similar to the paper specs.