As a warning, you do not want to use generic muffler hangers because often it will cause the muffler to hang too low and it will be scraping at every driveway you encounter. Eventually they will also become loose and allow the entire muffler system to shake around when you go over bumps. Trust me, the previous owner of my car was cheap and this is what happened.
Also, you do not want to use a generic, universal catalytic converter (like Carsounds) not only because of its inferior quality, but because it does not come with the necessary downpipes. While factory cats costs over $1,000 and are overbudget for most of us, their stock replacements cost one third the price and include the downipes that run from the cat to the exhaust manifold. Furthermore, since universal cats don't have the downpipes, the old cat must be sawed off and the new one welded back on. While the weld itself is quite strong, the areas surrounding the weld lose structural integrity and will eventually crack, causing an exhaust leak and a bunch of annoying rattles (like mixing balls in a spray can). Again, the previous owner went cheap and this was the result.
The oxygen sensor port on most 325i catalytic converters is located on the downpipes inside the engine bay, unlike the 325e (which is plugged directly by the cat). Your original O2 sensor can simply be plugged into the new port with the existing wire, no splicing is necessary. Do not use the 325i O2 sensor because that is specific to the Motronic 1.1/1.3 ECUs and will send an improper signal for the eta's Motronic 1.0 unit.
If you plan on adding the exhaust manifolds, you can also take the time to paint them before installation. I simply used a can of silver high-temp (1200 degree) paint from my local Kragen. I sanded all the rust off it, masked off the inlets, and sprayed three coats on it. After they dried I put them in the oven for an hour to allow the paint to adhere to the surface. If your wife/mom/chef is cooking some ham in the oven, I do not recommend sticking the manifolds in with them. If the manifolds are really dirty inside, then the entire house will smell like exhaust gas, so perhaps you'll want to clean them out beforehand. (You will also want to clean the oven out afterwards with oven degreaser. I did to avoid a lecture.) If you don't stick it in the oven, the paint may burn off or crack from the heat generated by the engine.
You can also paint the muffler using the same method, although sticking it in the oven is probably out of the question. Alex Stella, who I bought the muffler from, painted it flat black for me and it looks fantastic.
A used 325i factory exhaust cost me around $100, courtesy of Alex Stella. I bought a DEC catalytic converter from All BMW Parts for $345 (included necessary bolts and gaskets), while the exhaust manifolds were $100 from the junkyard. All the exhaust installation parts can be bought from a Worldpac affiliated company (like www.alloembmwparts.com or www.bimmerparts.net) for around $50. If you don't plan on swapping the manifolds, then you save an additional $35 because you don't have to buy the gaskets.
You are probably wondering why I didn't buy a Supersprint for that cost. Well, as good as the Supersprint is, it only has one pipe, as do all the other aftermarket mufflers for the eta. If you plan to do further upgrades to your engine besides an intake and a chip, then I recommend doing the swap. Otherwise, you are probably better off just buying a regular eta-specific muffler.
I actually ended up buying a Supersprint 325i exhaust in June 2002. The only reason why I bought it was because my friend was selling it cheap and because my stock exhaust "disappeared" after I installed the Racing Dynamics rear apron. The stock exhaust also burned a large hole in the apron, but it turns out that the Supersprint was no better. That's okay, it sounds very nice :).
Downpipes from manifold
Muffler w/dual pipes
Painted manifold before installation
Painted exhaust manifold
Supersprint 325i exhaust