I am an UIUC alumni 
for SAT ah...I think around 1400 to 1500 shud be enuff to get into
UIUC le...cos dats wat I got last time...I tot the total score is
1600? How come can become 1800 de? But I took the test in year
2001, so there might have been changes to the system 
Abt the A Levels grade, not too sure abt dat, cos I am from
Malaysia so they looked at my SPM results instead, but I think it
depends on what course you are applying for. If you are applying
for Engineering, definitely need to get at least B in Maths and
your Science subjects lor...
Then need to see your TOEFL score as well...
Ivy Leagues universities are definitely a lot harder to get into,
one of my frens who got into MIT have a score of 1590/1600. But
Northwestern shud be able to get in wif score around 1500/1600,
dependent on your A-Levels results as well and what course you are
applying for. Certain courses in the same universities are harder
to get into compared to the others, esp those that are ranked very
high. For instance, it is harder to get into the Electrical
Engineering course in UIUC compared to the Business course, cos the
EE course ranked higher than the Business course.