upgrading the memory doesn't void the warranty. thank god computer manufacturers have the sense to do that.
as for finding the right part,
Newegg.com - Crucial 4GB (2 x 2GB) 204-Pin DDR3 SO-DIMM DDR3 1066 (PC3 8500) Dual Channel Kit Laptop Memory Model CT2KIT25664BC1067 - Laptop Memory . bookmark it
extended warranties are, as a rule, rip-offs. the vast majority of computers work perfectly fine, and if they break, they break during the standard manufacturers warranty (usually a year). HOWEVER, that doesn't mean you shouldn't get one. If you know absolutely that you wont be able to afford a new computer should this one break, you should consider it. another option, which is cheaper, is to buy the laptop using a credit card that automatically puts a warranty on these kinds of purchases. if you dont know if your credit cards offer this, i would call them and ask.