I just spent $119 with shipping for 1GB from www.mushkin.com
for my 600m. Installation consists of turning the laptop upside down, loosening one screw to remove to cover plate, taking each index finger and popping the side springs off the SODIMM chip, and then carefully pushing the new SODIMM into each slot and pushing down gently to lock it in. I definately notice the difference between 256MB and 1GB. Whatever you do, buy decent chips something like corsair, ocz, or mushkin. I'm partial to mushkin because I've had good experience with their desktop memory and never had a problem with customer service.
For the geeks out there, even though the 600m is only advertised with PC2100 266MHz memory with a timing of 2.5-3-3, mine came with a PC2700 SODIMM clocking at 333MHz. I installed two 512 PC3200 Mushkin SODIMMS timed at 3-3-3 and 400MHz and they run just fine at 333Mhz. I ran through Memtest booted off my USB and there were no errors.
OK, I done geeking out, sorry.