Re: noticable differences between the 1.8T and 3.0? (Sunna)
IMO the 3.0L is much better and doesn't cost much more if you are looking at used 2002-2003 A4's. My GF just bought a 2002 3.0L A4 quattro w/ Tiptronic auto trans. We drove the 1.8t and it was a dog with the same options. True you can mod the 1.8t much easier, but just take a look at the factory numbers.
1.8t:
Horsepower (SAE net): 170 @ 5900 rpm
Torque: 166 lb/ft @ 1950-5000 rpm
0-60 seconds:
manual, FrontTrak®: 7.8
multitronic™, FrontTrak®: 7.8
manual, quattro®: 7.9
Tiptronic®, quattro®: 8.5
EPA fuel economy estimates(city/highway)
manual, FrontTrak®: 21/29
multitronic™, FrontTrak®: 20/29
manual, quattro®: 21/29
Tiptronic®, quattro®: 19/28
3.0L:
Horsepower (SAE net): 220 @ 6300 rpm
Torque: 221 lb/ft @ 3200 rpm
0-60 seconds
multitronic™,FrontTrak®: 6.9
manual, quattro®: 6.9
Tiptronic®, quattro®: 7.7
EPA fuel economy estimates (city/highway)
multitronic®, Frontrak®: 19/27
manual, quattro®: 18/25
Tiptronic® quattro®: 17/25
The 1.8t gives up 50 hp and 55 lb/ft to the 3.0L. This costs a tick less than a second 0-60. Also, the 1.8t only get 2-3 mpg better than the 3.0L. IMO that's a small price for 50 hp. The 1.8t is a GREAT motor, but the A4 is a pretty heavy car. You could actually call it a pig with the added weight of quattro. The 1.8t just doesn't have the displacement to lug that weight around and the turbo can't bridge the gap from 1.8 up to 3 liters. Again, you can mod the 1.8t, but what happens to your mileage advantage then? The 3.0L A4 is a great all-around car right out of the box. No mods needed and warranty in tact.
Good luck finding a 3.0L A4 though. There weren't many sold vs. the 1.8t. We had to travel to Chicago from Detroit to find what we wanted for the right price.