A solid performing SDD should outstrip U320 I imagine. Sandforce controllers seem the par for performance in this sector. I got 2 Intel drives (80GB G2)in RAID0 and I'm quite happy, but their writing performance is not as good as a Sandforce offering, not that the writing workload is ever more than downloads or installing apps, 175MB/s should be more than enough. The newer C300 controller is good too. I guess it depends on your budget and how much performance you actually want.
well a RAID0 of 2 Cruical C300 drives would be about as fast as you'd get without getting just stupid i think. 3 drives wont scale like 2 and you might even be hitting the bandwidth saturation on the bus with 3. I've heard theres a limit of about 667MB/s on a southbridge hdd controller??