I have done this, I have 1GB in my iMac. Are you familiar with computer hardware? If not then just go for the 512MB and then install the SO-DIMM yourself. It was kind of scary opening my iMac, as I didn't want to hurt anything, and I've been A+ certified for years
And of course the ultimate issue is money. You can get the same module from most other retailers for way under half the price. But they don't "Factory Install" it for you, which means that as soon as you open that sucker, your warranty is VOID.
Of course if you choose to do this on own, which I did, make sure you wait until AFTER you complete the installation before you become inebriated
Bottom line is, it's not hard, there are very good instructions on doing it, and if you have the patience to take it slow, and do it to the "T", then you should be alright. You will also save yourself a substantial amount of dough to boot.