20 gigs is too little. If you get a pro you get 60 gigs which should be fine. I can have a couple of games installed. The installs can be from about 5-8 gigs depending on the game. Installs take about 10-15 minutes so I only install games that I am currently playing.

They have deals like the Resident Evil bundle where you get a 120 gig which would totally rock if you were getting the game anyways. Rumor is that they are making a larger drive.

BTW harddrive installs are optional if you didn't know.  So as I said 60 gigs should be plenty for most people.

The games you've listed are all very well chosen.

The chances of becoming a Red Ring Of Death are very small now due to the new gpu/cpu chips. It might still be out there, but well you get a 3 years RROD warranty.

Whatever you do, never install Halo 3 on the HDD. It's the only game where the loading times get longer. (It already transfers data to the HDD while playing, so an installation slows it down)

You can delete and redownload. I have a 120G and I have 33G left, they get filled up pretty quickly ;) go for the Pro 60 .. thats your best bet.

I have a 20GB 360 and I have about 7 gb left. I have about 20 Arcade games purchased, so unless you are planning of installing full games, 20 is fine.

 Yes... all purchases are account based though they have to be registered to a console for offline play

Mass Effect should be your first priority.  Play the game and install it to the harddrive to minimise pop-in and frame-rate drops.

A truly awesome experience.

Get the 60 gig Hard Drive (or bigger), no matter what. Load times in games (Resident Evil / Street Fighter 4 load times are non-existent) are greatly reduced when saved to the HD, and the system runs cooler and as quiet as a mouse's fart.

I don't have a X360, but I do have a 80GB PS3. Mine has 20GB left and I've only had it since November.

I have about 10GB of game installs and DLC, 10GB of videos (all free, most are trailers, and a few special feature downloads), and 40GB of demos (I never get rid of them)

So yes, you can use the 60GB pretty fast, but in my case almost all of it can be deleted since I hardly use it. I wouldn't recommend a 20GB as then you wouldn't be able to easily use the install option on the X360.

Well, after 2 years (purchased march 07) my 20 gig still has 6.9 gig of space left, but with DLC becoming more popular (maybe not popular, but more frequent) i would reccomend a 60 gig, that should set you up for life.