I just took the plunge and bought a 2001 330ci to replace my 1993 318is. It is a huge difference in terms of handling, performance, and comfort.
Comfort - As both coupes with sport package, I think it's a fair assessments to say it is a lot more comfortable in the 330, but I find that space is more limited in the 330. Also, visibility is my main complaint - I can hardly see out of the 330, especially in comparison with the 318 that had next to no blind spots. (HK= WAY BETTER SOUND)
Performance - I would suggest getting a 323 with sports over a 325 if there is a significant difference in price - with very little modifications a 323 can pull on a 325 (it's a very underated engine).
Handling - This is another complaint I have. 01 and 02 330s have a lighter steering rack (arguably for fairies who buy these cars for status) and don't have the same steering feel. I really like the capabilities in terms of handlings and the predictability and traction of the car, but get a 99-00 328 if you want the legendary rigid steering feel (assuming 03+ cars are out of your budget). It is a cheap fix with swaping out the rack (ie $800ish) but it's a PITA.
I can't really argue on reliability because I've only owned this car for about a week or so, and the appearance factor is a personal preference - not about to get flamed for arguing e46s looks better (which they do!
Well that's a basic breakdown from a personal point of view. There's a lot of other differences, but these are the main things I have noticed making the change from the same car you own now.