Some advice from a long-time mac user:

1. Don't worry about buying third party RAM.

Do NOT buy RAM from Apple. It is overpriced and unless someone else is paying, doing so is just plain stupid.

You'll hear stories about "kernal panics" and unexplained crashes but it's all hogwash. As long as you buy RAM that fits the specs for the machine that you are buying it for you'll be fine. That much said, stay away from generic RAM and go for those with at least some track record of reliability. Mushkin, Kingston and OCZ come to mind but the list is (obviously) much longer.

2. By All Means : Install the RAM yourself. It will NOT void your warranty.

Then again; if you're a complete technological doofus and are unable to tell a phillips-head screwdriver from a flat-head one; do yourself a favor and pay the technician already.

3. Don't buy RAM you don't need.

Why do you need 4 gigs? Are you going to be doing a lot of rendering and CAD work? May I suggest going for 2 Gigs and monitoring your page-outs on iStat Pro (do a google search for iStat Pro: it's free btw - as is Google).