What Nikon is comparable to the Cannon 550d or 600d and what drawbacks (or good points) does it have compared to the canon?
The Nikon D5000 is roughly equivalent to the 550d. The Canon has a higher megapixel rating, but most other specs are pretty similar. The D5000 has now been superceded by new models: the D3100 and D7000.
Reams and reams have been written on the Canon vs. Nikon debate, and I'm not about to add to that (other than to say, GO NIKON!); try visiting DPReview.com for in-depth, impartial reviews.
I would say a rough equivalent would be the D3100 or the D5000, they are around the same price point and have similar features.