RB/GH ??? From an XBox Shopper

Okay, I'm probably REEEALLY late to the party asking this question, but whatever.

I'm currently shopping around to buy my first 360. Obviously not a hard-core player. I'm weighing my options between picking up a refurbished 20GB system for $270 or a new 60GB for $350.

So here's my question:

How much memory does the average downloadable song from Rock Band or Guitar Hero take up? Are we talking 5 MB? 50? 500?

I definitely a casual gamer, who doesn't see myself buying more than maybe a dozen games total for the system, so I won't be stuffing the Box full of other stuff. Would 20GB suffice for RB, GH, and maybe one or two more games like Penny Arcade or any FFXIII content that's gonna show up? Or would I be stupid to get anything less than 60GB, even if it's gonna cost me $80 more?

Wait a couple of weeks. Credible rumors point to the 60GB dropping at $299 in September. Then, get that. 20 sounds like it would suffice at first, but the usable space is more like 12, and that can fill up quick, especially with RB/GH DLC. TRUST ME. I thought before I bought Rock Band "Oh cool, I'll pick up a few songs here and there." In six months I've ended up in for about 60 songs with more on the way. They're only around 50 MB give or take depending on the song, but it adds up fast.

I think I'm patient enough to wait for the rumored $299 60 gig to go down even further for post-Christmas frenzy.

From what I saw, my friends' 20 gig systems filled up pretty quickly with the each demos being around 1gig big.

Yeah. Definitely going to wait.