My 2 years old iMac is doing great, but I was thinking about upgrading the RAM to 4GB. It's a 20" Intel Core 2 Duo 2.4GHz with 2Gb of 667MHz RAM (2x1Gb). It doesn't feel slow, but would I get any noticeable improvements by increasing to 4Gb? Would upgrading the RAM invalidate my Apple Care which still has another year to go? Thanks.