Hello and welcome to Mac-Forums!
The 24" iMac is one sweet machine. I don't think you'll be disappointed with your choice. OSX is a fantastic computing experience and the iMac is pure eye candy.
Hard drive specs are purely subjective. If you have a lot of data (ie. movies, photos, projects etc.) you may opt for a larger hard drive. If you're just a general internet, word processing, family photos type of person, the standard size will be plenty.
External drives are a very popular option. I would recommend at least one external drive for Time Machine (new auto-backup function in Leopard).
I ran my iMac with 1 GB of RAM for a while and it ran just fine. I think 2 GB is your best option. It's a good balance between speed and cost effectiveness. You will notice an increase in performance with 2 GB. Programs will load faster and it will be smoother overall.
I would recommend that you buy RAM form a third party. Apple charges way to much for RAM. You can get an additional 1 GB stick for 20 to 30 bucks. It takes maybe 1 minute to install. A monkey could do it. No, it won't void your warranty.