Unless you can get a really good deal on a used housing, I am doubtful you'd find a housing for your rebel xt within the $400 budget. Between housing, port, and strobe, I think you're definitely over the $1k mark, even used. But maybe I'm wrong and someone else out there will correct me!
I would definitely look at used gear, though. The SeaLife DC600 goes for around $500 new (no strobe ~$700 with strobe), and Sea and Sea has a competing product, the DX 860G, for around the same price. I'm sure one of these will wind up used on eBay that will fall in your budget. These are point and shoot systems though, so bear that in mind also. Also, if you're willing to pass on a strobe for a while, the PT series of Olympus housings and cameras are all over eBay and can be had for within your budget also.
Older systems are always selling for cheap, too. I was able to buy an Olympus 5050 camera, light and motion housing, strobe, sync cords, arms, pelican case and other accessories for $650 on eBay, saw a similar one with less for $500. These rigs ran for around $2k back in their day!
Be prepared, though, once you enter the world of UW photography, you're going to wind up spending a whole lotta cash.

I love this hobby, but it ain't cheap!