You have several options.
If U160 is fast enough for you and you only need a single channel, grab a ServeRAID 4Lx if you can still find one. It's a single channel full-blown RAID controller. You can still use the two channels of the onboard controller for, say, a tape streamer.
If you need U320 you have several choices. ServeRAID 5i, 6i and 6i+ are zero channel RAID controllers. They don't have any SCSI connectors but they turn the onboard SCSI controller into a dual channel RAID controller. Looks nice but it sucks if you want to use one of those channels for something else than disks. I've had headaches getting backup software to work correctly on both Netware and Windows. In the end we just added a separate U160 or U320 SCSI controller for the tape streamer. 5i and 6i are LSI-based while 6i+ is an Adaptec. 6i+ should also work with the x225's onboard LSI controller some IBM support person told me but check IBM's ServerProven hardware compatibility list.
Better but more expensive than those zero channel controllers is the 6M, a true dual channel U320 RAID card with 128 or 256 MB cache. Sadly enough there's no single channel version of this card, like the U160 4Lx was a single channel counterpart to the dual channel U160 4Mx.
All those cards, whether they have their own connector or use the onboard SCSI, have 68-pin connectors. You say the server has hot swap drives - should be 6 bays in a 225 - so they'll be 80 pins. If you don't wish to buy IBM drives, be certain to get U320 80-pin drives even if you get a U160 controller. Some drive bays don't support U160 drives, only U320. Don't ask me why. I asked IBM and nobody could give me an intelligent answer. You will need to buy hard drive sleds from IBM to mount those drives in though. Another thing to look out for is the SCSI cable connecting the onboard controller to the backplane. Chances are it will be too short if you get anything but a zero channel controller. Order a longer one at IBM. It would be a shame if you have to install a controller but the cable is too short by a couple of cm. Been there, etc.
So, in short.
If U160 and one channel are good enough get a 4Lx (not a 4L).
If you need two channels U160, get a 4Mx (and not a 4M).
If you need U320 get a 6M.
Get a longer SCSI cable!
A zero channel controller may be less expensive but if you need to add a dedicated SCSI controller for a tape streamer, the price difference may become negigible. You can always use a full-blown RAID controller in any other machine, not so with a ZCR controller.
If you can give me the exact model - 8646-5AX for example - I can check on IBM's site if you want.