U320 is the speed of the drive. Its quite a bit faster than U160... but for one drive, U160 would probably be fine. Depending on what prices you find for either type, I'd say to go with a U320 card, just on general principles. I have an LSI based Tekram U160 card in my winbox right now. and I like it alot. They make good stuff.
Just about every U160 or U320 card I can recall seeing has been bootable., as far as I can remember. Go over to the
LSI website and take a look at their U320 stuff. Mmmmmmm a MegaRAID 320-4x. 4 channels of U320 with PCI-X connectivity and 128MB of cache..... Yummy. Of course, you'd need to sell an organ or 4 to afford one...