Search for posts by "tropical1" on this forum. He's probably the most knowledgeable overall Olympus user here, having been thru the E-3xx series up thru the E-5xx series and currently I believe shooting an E-3. Although he's stepped up for the E-3 he's been a big fan of the Olympus housings in the past and knows the strobe options, which (to my knowledge) boil down to fiberoptic cabling vs. any sort of electrical TTL signaling, with the possible exception of the Olympus branded strobes. So that means you'd still likely be using your DS-125's with sensors and the internal flash, I believe.
I would not "plan" on using either the 520 or 620 live view as your primary shooting mechanism though. Although faster than previous iterations from all I've read (with the exclusion of the E-330, which was the only Olympus E-series Liveview to use a 2nd dedicated sensor and therefore eliminate additional mirror movement) all of them will be probably approaching the delay of a point and shoot, in which case, why go DSLR vs just a newer point-n-shoot. (BTW, don't apologize for the 5050...from what I read it had fabulous optics for a p-n-s).
If you want to keep the DS-125 setup and use it cabled, and are already comfortable using an Ike rig overall, then my opinion would be to go with an Ike housing for whatever you get - that way you can cable and use iTTL. But if you're more interested in the form-factor of the Oly housings, you'll still be able to use the strobes with the sensors and arms I believe.
Other Oly users to search on are me (not that I have any experience to mention, just a whole lotta hot air), ce4jesus, and jlyle.
If you're really after "fast" live-view from a DSLR, then in my opinion your options are the Oly E-330 (not a great option, I love mine but they're getting a bit long in the tooth and finding the housing can be tough from either Ike or Olympus at this point), the Sony offerings which use an extra mirror-tilt option and 2nd sensor (A300 and A350 I believe), or the new "micro-4/3rds" offerings from Panasonic which eliminate the mirror entirely and are entirely based on electronic viewfinding. If you're comfortable with your more current shutter lag times though, then the 520/620 would likely be fine for you. If I were buying today between those two, I'd go 620 since it incorporated some of the E-30's newer hybrid focus features. Just remember you only get those with "some" lenses, not all of them (e.g. the 14-54 v 2, not the original 14-54, etc.)