Hey Kev!
As long as you buy your RAM from a reliable supplier like OWC or Datamem and get the correct DIMMS according to their profiler specs, you'll be fine.
Just hang on to the stock DIMMs, in case you do need to take your machine in
some day.
It's not a warranty problem, but it helps to rule out a third party RAM issue.
I know you were trying to stick to a budget, but I'd probably go for the Core Duo 2 models at this point, even if you need to drop down a screen size.
Education price on a NEW 17" Core Duo 2 1.83 GHz is now starting at $899.00
$1099.00 for the 2.0 GHz 17" and $1399 for the fast 2.16GHz 20"
At least you have a lot more options.
You could also buy NEW Core Duo 1's on closeout.
Decisions Decisions.