Welcome to the madness.
My immediate guess re: quality would be yes, you should see further image improvements going to the Nikon DSLR vs. the Micro4:3rds camera. Sensor size is part of it, so megapixel count (resolution) and sensitivity (ability to go higher ISO with less noise, or just improved dynamic range within the frame regardless) will both improve. While the m4:3rd contrast detect autofocus has been getting faster and faster, DSLR phase detect AF and shutter response should be very fast as well, making your ability to not end up with 'fish butt' improved as well. Don't forget that you'll have to get used to shooting with the optical viewfinder vs. a rear screen, and this will take some getting used to. The equipment does not in the end "make" or "break" the photo - more like it provides some headroom. So while the potential is there, it may still take some getting used to, and your strobe light doesn't magically get more powerful with a DSLR so don't expect that higher ISO alone helps you keep colors further into the frame, and that sort of thing.
All Ike ports are "normed" if by that you mean same diameter at the mounting, work in any DLSR housing. They do have different lengths of course to accomodate different lenses and different lenses will also need different zoom gearing parts (two types come with the housing purchase, a normal and 'lower profile' clamp/sleeve for larger diameter lenses).
You didn't say what lenses you were using on the E-PL1, but I suspect that you could start with a reasonable mid-range lens, something in the 17-50 or 17-70mm type range. I don't know Nikon lenses but you might try finding a used Sigma 17-70mm without the HSM or OS....if you pay over $350 you've probably paid too much for that one in particular. http://www.sigmaphoto.com/shop/17-70mm-f28-45-dc-macro-sigma . Try eBay, KEH.com, even B&H and Adorama have "used" shops that sometimes have these lenses. According to Ikelite it only "works" in their 8" dome but I've been experimenting with it in the 6" dome (5503.80) with what seems like reasonable results in the pool at least (without a 2x diopter the corners might be a tiny bit soft and it vignettes a little bit at maximum angle, and the lens barely fails to touch the dome interior at full extension/zoom. With a +2 diopter the corners are sharper but of course you lose a bit of the wide angle and the extra length can crash into the dome interior at about 60mm length). The molded 6" domes only list for $200 vs. the $550ish that the 8" dome plus extension will end up running. But that's just one suggestion- I'm sure you can find other used Nikon lenses that would be in a reasonable focal length range and allow you to use a cheaper dome. And of course if your real interest is wide angle or macro...then just about all that paragraph was a waste of your time. 
Also don't forget to check out the classified forums here for used Ike domes - there do tend to be several at a time, usually the 5503.50 or .55 type standard lengths for mid-range or 'standard' zooms.
Remember also that you may not be able to raise the Nikon's flash inside the Ike housing, so more than likely you'll be firing your strobes with 'manual' mode, triggered from an electrical sync cable, unless you switch to Ikelite's strobes in which case you get full TTL metering. That may be different than you were probably used to with the Olympus.
