If one is not planning to lower their M3, which staggered CSLs would look better, 18"s or 19"s? How much does acceleration suffer with 19" wheels? Next, what are some good aftermarket exhaust systems that eliminate (or mostly) that nasty (imo) raspy sound the E46M3 engine makes (from what i've heard in the 2000-3000RPM range)?

I would definantly go with the 19"s. They look a LOT better. But if you're worried about acceleration i would go with the 18's... They do not make that much of a difference on acceleration because the weight is about 1lbs difference between each wheel for the 19"s. If you have any questions regarding the wheels give me a call or email!

As for your second question about the rasp... Though some people might disagree i think the best bang for the buck is the Generation 2 Rasp-x by UUC motorwerks...

Borla also makes a mid section rasp eliminator....about $200 cheaper than the UUC one

But a good rasp pipe will eliminate the noise you hear...no muffler or exhaust will eliminate that sound

Go 19s on the E46. the E36 is fine with 18s but the e46 looks so much better with 19s, especially when you go with something like IForged or HRE with a lip. (buy mine ;P)
http://forums.bimmerforums.com/forum/showthread.php?t=955809
I am currently running the CSLs too and they look great. Bob at Bimmerperformance store has some great knowledge of midpipes and the rasp although I am only running the SS Race at this time.

Thanks for the advice fellas!